IrfAIN  LlERARY-AGRfCU 


HANDBOOK 

OF 


METEOROLOGY 

A  Manual  for  Cooperative 
Observers    and   Students 


BY 


JACQUES  W.  REDWAY 

FELLOW,  AMERICAN  METEOROLOGICAL 
SOCIETY 


NEW  YORK 

JOHN    WILEY    &    SONS,     Inc. 

LONDON  :      CHAPMAN    &    HALL,     LIMITED 
1921 


COPYRIGHT,  1921 

BY 
JACQUES  W.  REDWAY 


PRESS  OP 

BRAUNWORTH  &  CO. 

BOOK  MANUFACTURERS 

BROOKLYN,   N.  Y. 


PREFACE 


This  text-book  has  been  prepared  for  the  use  of  cooperative 
observers  and  for  the  instruction  of  students  in  meteorology  and 
aeronautics.  It  is  essentially  a  laboratory  manual. 

Part  I  is  an  elementary  synopsis  of  the  general  principles  of  air 
science,  but  is  not  wholly  restricted  thereto.  The  subject  of 
atmospheric  transparency  and  the  principles  of  visibility  may 
not  be  logically  a  part  of  meteorology,  but  it  is  one  of  great  impor- 
tance, not  only  to  air  men,  but  also  to  every  one  engaged  in 
transportation.  The  chapter  on  the  dust  content  of  the  air  in 
part  summarizes  the  researches  of  author  in  this  field. 

Part  II  is  descriptive  of  the  instruments  used  in  meteorology 
and  the  construction  and  care  of  them.  The  methods  of  obser- 
vation discussed  is  a  resume  of  the  experience  of  many  observers 
covering  a  period  of  half  a  century.  This  part  of  the  text  is 
specifically  designed  for  the  use  of  cooperative  observers  and 
students. 

The  Appendix  contains  conversion  and  other  useful  tables 
that  are  not  included  in  the  Circulars  of  Instruction  published  by 
the  U.  S.  Weather  Bureau.  English  units  of  measurement  are 
used  throughout  the  book,  but  the  metric  equivalents  are  used 
when  necessary. 

In  the  final  revision  of  the  text,  the  author  desires  to  express 
his  appreciation  for  the  counsel  received  from  James  H.  Scarr, 
forecaster  in  charge  of  the  New  York  City  Weather  Bureau 
Office,  and  to  J.  H.  Kimball  of  the  same  office  in  charge  of  marine 
work.  Acknowledgments  are  due  to  Dr.  Charles  F.  Brooks, 
editor  of  the  Monthly  Weather  Review,  for  the  cloud  photographs 
used,  and  also  for  his  critical  knowledge  of  cloud  science;  to  Dr. 
W.  J.  Humphreys  for  the  use  of  illustrations  in  his  Physics  of  the 
Air;  and  to  Charles  Scribner's  Sons  for  the  use  of  illustrations 
taken  from  Redway's  Physical  Geography, 

METEOROLOGICAL  LABORATORY 
Mount  Vernon,  N.  Y. 

iii 

4G5041 


TABLE   OF   CONTENTS 


PART  I:  GENERAL  PRINCIPLES 

PAGE 

I.  THE  ATMOSPHERE  AND  ITS  CONSTITUENTS i 

II.  FORMS  AND    PROPERTIES  OF   MATTER   IN    RELATION    TO   ME- 
TEOROLOGY    10 

III.  HEAT:  ITS  NATURE,  PROPERTIES  AND  DIFFUSION 16 

IV.  THE  AIR:  DISTRIBUTION  OF  WARMTH 23 

V.  THE  AIR:  DISTRIBUTION  OF  PRESSURE 39 

VI.  THE  AIR:  MAJOR  CIRCULATION:  LOCAL  WINDS 49 

VII.  THE  MOISTURE  OF  THE  AIR:  EVAPORATION  AND  CONDENSATION.      60 

VIII.  THE  MOISTURE  OF  THE  AIR:  FOGS  AND  CLOUDS 68 

IX.  THE  MOISTURE  OF  THE  AIR:  PRECIPITATION 93 

X.  ATMOSPHERIC  ELECTRICITY:  OPTICAL  PHENOMENA 108 

XI.  THE  DUST  CONTENT  OF  THE  AIR -.   125 

XII.  THE  PRINCIPLES  OF  ATMOSPHERIC  VISIBILITY 133 

XIII.  THE  DAILY  WEATHER  MAP:  STORMS 146 

XIV.  FORECASTING  THE  WEATHER:  FOLKLORE 164 


PART  II:  INSTRUMENTS  AND  MEASUREMENT 

XV.  THE  MEASUREMENT  OF  TEMPERATURE:  THERMOMETERS 177 

XVI.  THE  MEASUREMENT  OF  PRESSURE:  MERCURY  BAROMETERS 193 

XVII.  THE  MEASUREMENT  OF  PRESSURE:  ANEROID  BAROMETERS 206 

XVIII.  THE  MEASUREMENT  OF  HUMIDITY:  HYGROMETERS 215 

XIX.  THE  MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES;    SNOW 

MEASUREMENT 222 

XX.  THE  MEASUREMENT  OF  WIND  VELOCITY:  ANEMOMETERS 234 

XXI.  THE  MEASUREMENT  OF  SUNSHINE:  SUNSHINE  RECORDERS 245 

XXII.  APPENDIX:  CONVERSION  AND  OTHER  TABLES 253 


METEOROLOGY 


PART  I 

CHAPTER    I 
THE   ATMOSPHERE:    ITS   CONSTITUENTS 

Meteorology  is  the  science  of  the  air.  The  air  is  the  outer 
shell,  or  layer,  of  the  earth;  hence  it  is  called  the  atmosphere, 
a  Greek  word  meaning  "air-sphere."  The  movements  of  the  air 
(the  winds)  and  the  variable  proportion  of  water  vapor,  mingled 
with  and  forming  a  part  of  it,  exert  a  profound  influence  which 
affects  the  climate,  habitability  and  civilization  of  the  earth. 

For  the  greater  part  the  movements  of  the  air  and  the  varia- 
tions in  the  proportion  of  moisture  are  the  results  of  changes 
in  temperature.  The  fundamental  study  of  the  physics  of  the 
air,  therefore,  concerns  the  problems  of  variations  in  tempera- 
ture and  the  far-reaching  results  of  those  variations. 

Composition. — The  atmosphere  consists  of  a  mixture  of 
gases  of  which  oxygen,  nitrogen  and  the  argon  group  constitute 
about  98  per  cent.1  The  composition  varies  very  slightly  so  far 
as  these  are  concerned;  but  the  proportion  of  water  vapor  and 
of  the  other  components  varies  materially.  The  first  of  the  two 
tables  which  follow  is  the  average  of  many  analyses  made  by 
Rayleigh  and  Ramsay;  the  second  is  on  the  authority  of  Hum- 
phreys. Other  analyses  show  slight  differences  that  indicate 
actual  differences  in  proportion  rather  than  errors  in  analyses. 

1  The  argon  group  consists  of  argon,  krypton,  xenon,  and  neon.  Two 
other  very  rare  elements,  coronium  and  niton,  are  probable  constituents  of 
the  air. 


T*  IE;  ATMOSPHERE:     ITS   CONSTITUENTS 


The  values  of  all  except  the  first  three  are  variable;  that  of 
floating  dust,  hydrogen,  and  helium  is  empiric  and  calculated. 

The  values  determined  by  Humphreys,  are  those  of  air  from 
which  the  water  vapor  has  been  removed  —  that  is,  of  dry  air. 


& 


Altitude  in 
meters  and  MfleS 


B 


V 


» 


II 


20 
12.4 


11 

6.8 


\ 


^n 


Reg-ion 


F  ydrogen 


Reg 


Nitrogen 


of  Cqnstan 


-67.1  F. 
-55 


of 


Convetion 


Tern 


>erature 


10        20 


30 


vlpor 


Pressure 
in  Inches 
L0002 


Hdliutn— 


ygen 


70 


90 


0004 


0005 


.007 
.340 
.61 


1.61 


7.71 

29.92 


40        50        60 
Volume  Per  cent 

After  Humphreys  (Physics  of  the  Air). 


The  distribution  of  the  constituents  of  the  air. 

The  foregoing  represent  the  proportions  at  the  surface  of 
the  earth.  The  proportions  change  with  increasing  altitude. 
The  nitrogen  disappears  at  a  calculated  height  of  84  miles;  the 
oxygen,  at  about  60  miles.  Water  vapor  is  calculated  to  exist 
at  an  altitude  of  60  miles,  but  it  is  not  observable  above  7  or  8 
miles.  Carbon  dioxide  is  not  observable  above  an  altitude  of 
2  or  3  miles;  theoretically  it  may  extend  to  a  calculated  height 
of  more  than  15  miles.  The  proportion  of  hydrogen  and  helium, 


DEPTH  OF  THE  ATMOSPHERE 


on  the  other  hand,  increases  with  altitude,  and  they  probably 
form  the  outer  layer  of  the  atmosphere.1  Because  of  their  light- 
ness it  is  not  unlikely  that  hydrogen  and  helium  are  gradually 
escaping  from  the  earth. 


Constituents 

Parts  in  one 
million  of  air 

Nitrogen 

771,200 

Oxygen                 

206,600 

Argon  group  (approximately).  . 
Water-  vapor  

7,900 
13,953 

Carbon  dioxide   .  .            .... 

336 

Ozone  . 

12 

Nitric  and  nitrous  oxides  
Ammonia 

8 

Dust,  hydrogen,  helium.  ..... 

i(?) 

Depth  of  the  Atmosphere. — Observations  on  the  twilight 
arch  indicate  that  at  a  height  of  40  miles  above  sea  level  the 
air  has  a  density  sufficient  to  refract,  reflect,  and  diffract  light.  A 
measurement  of  the  parallax  of  a  meteor  seen  by  two  observers 


Constituents 

Per  cent 

Nitrogen                         

78.03 

Oxvffen 

20  QQ 

Argon             

0.94 

Carbon  dioxide  ...        

O.O3 

Hydrogen 

O   OI 

Neon  
Helium 

0.0012 
o  .  0004 

at  different  stations  indicates  the  existence  of  air  at  a  height  of 
200  miles.  Actual  measurements,  however,  have  not  extended 
much  higher  than  20  miles,  the  height  to  which  sounding 
balloons  have  reached. 

At  an  altitude  varying  approximately  from  6  to  7  miles, 
according  to  latitude  and  also  according  to  the  season,  a  plane 
of  contact  occurs  which  apparently  separates  an  upper  from  a 

1  The  foregoing  are  on  the  authority  of  W.  J.  Humphreys. 


4  THE  ATMOSPHERE:    ITS   CONSTITUENTS 

lower  shell  of  air.  Below  this  plane  practically  all  the  local 
movements,  especially  the  upward  and  downward,  or  convec- 
tional  movements  of  the  air  occur.  The  lower  or  convectional 
shell  is  the  troposphere;  the  upper  shell  is  the  stratosphere. 

Constituents  of  the  Air. —  Nitrogen,  the  constituent  of  great- 
est volume  at  the  surface  of  the  rock  sphere,  is  very  inert.  It 
does  not  combine  with  the  oxygen  of  the  air,  except  in  very 
minute  quantities  when  influenced  by  lightning  discharges. 
The  nitrogen  of  the  air  is  now  used  in  the  manufacture  of  am- 
monium nitrate,  the  basis  of  certain  explosives.  Nitrogen  is 
the  chemical  base  of  nitric  acid,  HNOs,  and  of  several  other 
oxygen  compounds.  It  is  a  constituent  of  ammonia  gas,  NHa, 
and  of  cyanogen,  CN,  all  of  which  enter  into  the  structure  of 
many  thousand  other  compounds.  Many  of  these  compounds 
are  very  unstable;  hence  the  rapid  decomposition  of  animal  and 
vegetable  compounds,  commonly  known  as  putrefactive  decay. 
The  instantaneous  dissociation  of  the  nitrogen  constituents  of 
such  compounds  as  nitroglycerine  and  tri-nitrotoluol,  or  TNT, 
give  to  such  compounds  their  value  as  explosives. 

Atomic  weight  13.93;  sp.  gr.  .971;  temperature  of  liquefac- 
tion —  231°  F  (—146°  C)  at  35  atmospheres  pressure.1 

Oxygen  is  the  active  chemical  element  of  the  air.  It  unites 
readily  with  pretty  nearly  every  other  chemical  element.  Its 
union  with  carbon  is  the  ordinary  process  of  combustion.  Iron 
wire  in  free  oxygen  burns  about  as  freely  as  a  match  in  the  open 
air.  The  oxygen  of  respiration  oxidizes  the  impurities  of  the 
blood. 

The  percentage  of  oxygen  is  slightly  greater  in  the  air  of 
northerly  winds  of  the  north  temperate  zone  than  in  southerly 
winds.  It  is  slightly  below  normal  over  cities,  as  compared  with 
open  spaces.  In  crowded  auditoriums  the  proportion  of  oxygen 
sometimes  falls  to  20  per  cent;  in  mine  tunnels  it  is  sometimes 
as  low  as  1 8  per  cent.  Candles  burn  with  difficulty  with  the 
oxygen  content  at  1 8  per  cent;  and  human  life  cannot  long 
exist  with  the  proportion  of  oxygen  as  low  as  17  per  cent. 

Atomic  weight  15.88;  sp.  gr.  1.106;  temperature  of  liquefac- 
tion —  182°  F  (119°  C)  at  51  atmospheres. 

Carbon   dioxide    (carbonic   acid    gas),    CO2,    is   the   heaviest 

1  The  temperature  and  pressure  of  liquefaction  of  the  gases  mentioned  in 
this  chapter  vary  slightly  according  to  different  authorities. 


THE  WATER  VAPOR  OF  THE  AIR  5 

gaseous  constituent  of  the  air.     It  is  derived  from  carbon  in  the 
ordinary  process  of  combustion: 


It  is  also  derived  from  various  hydrocarbons  of  rotting  vegeta- 
tion by  dissociation  and  combustion,  as,  for  instance,  methane 
(marsh  gas)  : 


The  normal  proportion  of  carbon  dioxide  in  the  air  is  about 
3.3  parts  in  10,000  of  air.  In  manufacturing  districts,  where 
coal  is  used  for  power-fuel,  the  proportion  is  greater.  In  the 
bracing  air  of  a  cold  wave  it  is  materially  less.  It  is  less  during 
winter,  when  the  temperature  is  below  freezing  and  the  ground 
is  snow-covered,  than  in  summer.  Over  the  land  the  proportion 
is  slightly  greater  at  night  than  in  the  day-time,  and  during 
foggy  weather  it  is  materially  greater  than  in  dry  weather. 

In  theaters,  churches,  schoolrooms,  and  poorly  ventilated 
rooms  the  proportion  of  carbon  dioxide  may  be  as  high  as  12 
parts  per  10,000  of  air;  occasionally  it  is  even  greater.  Breathed 
air  is  harmful,  not  so  much  on  account  of  its  carbon  dioxide 
content  as  on  account  of  the  presence  of  products  of  putrefac- 
tion. Although  carbon  dioxide  exists  in  the  air  at  a  calculated 
height  of  15  miles,  the  proportion  decreases  so  rapidly  that  it 
may  be  disregarded  as  a  component  of  the  air  above  the  height 
of  i  mile. 

Sp.  gr.  1.53;  liquefies  and  solidifies  with  moderate  pressure 
at  ordinary  temperatures. 

Water  vapor  in  varying  proportions  is  a  constituent  of  the 
air.  The  maximum  proportion  depends  chiefly  on  temperature. 
Thus  at  30°  F  there  may  be  nearly  2  grains  by  weight  of  water 
vapor  per  cubic  foot;  at  70°  F,  there  may  be  nearly  8  grains. 
There  may  be  less  in  either  case,  but  there  cannot  be  more; 
any  excess  will  be  condensed.  When  the  maximum  proportion 
is  present  the  air  is  conveniently  said  to  be  "saturated."  l  The 

1  According  to  common  use  air  is  said  to  "contain  water  vapor"  or  to  be 
"saturated"  under  certain  conditions,  as  though  the  air  were  a  sponge, 
which  may  absorb  and  retain  water  up  to  a  certain  limit.  The  expression  is 
inexact;  but,  in  the  literature  of  meteorology,  inasmuch  as  the  water  vapor  is 
rarely  considered  apart  from  the  other  constituents  of  the  air,  expressions 


6  THE  ATMOSPHERE:    ITS  CONSTITUENTS 

table,  p.  280,    shows   the   maximum  weight  of  water  vapor  at 
different  temperatures. 

Although  the  proportion  of  water  vapor  mingled  with  the 
air  differs  from  time  to  time,  the  per  cent  of  total  volume 
decreases  from  the  equator  toward  the  poles.  The  average 
annual  per  cent  at  the  equator  is  2.63;  in  latitude  70°  it  is 
only  O.22.1  The  proportional  water  vapor  content  of  the  air  is 
commonly  expressed  as  "per  cent  of  humidity."  Thus,  with 
half  the  maximum  proportion  of  vapor,  the  humidity  is  50  per 
cent. 

Sp.  gr.  0.62;  "boils"  with  vapor  tension  equal  to  that  of  the 
air  at  sea  level,  at  212°  F  (100°  C);  solidifies  or  "freezes"  at 
32°  F  (o°  C). 

Argon  and  the  related  group  of  elements,  neon,  krypton  and 
xenon,  constitute  practically  8  parts  per  thousand  of  air.  The 
gases  of  the  argon  group  are  chemically  inert;  no  compounds 
with  other  elements  are  known  to  exist.  This  is  true  also  of  the 
other  elements  of  the  group.  If  they  have  any  specific  influence 
not  possessed  by  nitrogen,  the  influence  is  not  known. 

Atomic  weight  of  argon  39.88;  sp.  gr.  1.21;  liquefies  at 
—  184°  F  (—120°  C)  under  pressure  of  40  atmospheres. 

Hydrogen  is  the  lightest  of  the  chemical  elements,  and  the 
weight  of  its  atom  is  the  unit  of  atomic  weights.  Ignited  with 
oxygen  it  forms  water: 

2H+O  = 


Hydrogen  is  a  constituent  of  all  chemical  compounds  con- 
taining water,  and  of  the  various  hydrides  and  hydrates.  It 
occurs  in  the  lower  air  in  variable  but  very  minute  proportions 
which  may  be  due  to  the  chemical  dissociation  of  organic  matter. 
It  is  a  constituent  of  natural  gases,  and  of  certain  volcanic 

of  the  sort  are  convenient  and  will  be  so  used  in  this  manual.  In  a  given 
space,  whether  vacuous  or  filled  with  the  other  constituents  of  the  air,  there 
may  be  a  certain  number  of  molecules  of  water  vapor  at  a  given  temperature 
and  pressure,  and  no  more.  If  additional  molecules  are  added  the  excess 
will  be  "condensed"  and  become  a  liquid.  The  water  vapor  is  at  its  maxi- 
mum density,  and  also  it  is  "saturated,"  when  the  space  contains  all  the 
water  vapor  which  can  exist  therein  up  to  the  point  of  saturation.  Strictly 
speaking,  it  is  the  vapor  itself  and  not  the  space,  nor  the  air  which  is  "satu- 
rated." 

1  Hann  :   Lehrbuch  der  Meteorologie. 


HELIUM 7 

gases.  The  proportion  increases  as  the  height  in  the  air 
increases.  On  account  of  its  lightness  meteorologists  are  of  the 
opinion  that  the  rapid  movement  of  the  earth  in  space  is  con- 
stantly throwing  it  off  into  space.  At  all  events,  there  seems  to 
be  sufficient  evidence  that  it  is  the  chief  if  not  the  sole  con- 
stituent of  the  outer  part  of  the  atmosphere.  It  is  much  used 
for  the  inflation  of  balloons  and  airships,  being  about  15  times 
as  buoyant  as  air. 

Atomic  weight  I;  sp.  gr.  069;  liquefies  at  about  —375°  F 
(  —  226°  C)  under  a  pressure  of  15  atmospheres. 

Helium  is  another  inert  element.  It  is  a  constituent  of 
several  minerals,  including  pitchblende,  an  oxide  of  uranium. 
It  occurs  by  absorption  in  many  deep  rocks  and  also  in  the  gases 
that  escape  from  deep  springs.  Cottrell  discovered  it  in  the 
proportion  of  about  2  per  cent  in  certain  Texas  gas  wells.  Be- 
cause it  is  non-explosive  and  non-inflammable,  it  has  been  used 
in  the  inflation  of  balloons.  Its  buoyancy  is  about  92  per  cent 
of  that  of  hydrogen  and  it  does  not  readily  pass  through  balloon 
fabrics.  Because  of  its  lightness  and  also  its  high  molecular 
speed,  it  is  thought  to  occur  chiefly  in  the  outer  shell  of  the 
atmosphere — possibly  escaping  from  the  earth  altogether.  If 
it  plays  any  part  in  meteorological  phenomena,  its  influence  is 
not  known. 

Atomic  weight  4;  sp.  gr.  approximately  .128;  liquefies  at 
—452°  F  (—269°  C)  at  a  pressure  of  about  3  atmospheres. 

Nitric  acid  (HNOs)  and  ammonia  (NHs)  are  present  in  the 
air  in  very  minute  proportions.  Nitric  acid  is  most  readily 
detected  at  the  time  of  thunderstorms.  From  time  to  time 
the  proportions  of  these  substances  vary  greatly  from  the  pro- 
portions noted  in  the  table.  The  presence  of  ammonia  is  due 
probably  to  the  decomposition  of  organic  matter. 

Ozone  (Oa)  is  an  allot ropic  form  of  oxygen,  whose  normal 
molecule  is  62.  Ozone  possesses  a  pungent  odor  that  fre- 
quently is  discernible  at  the  time  of  nearby  lightning  discharges 
and  the  passage  of  high-potential  electric  sparks.  The  normal 
proportion  in  the  air  is  exceeded  many  times  over  during  thun- 
der-storms. 

The  proportion  of  ozone  varies  with  environment.  It  is 
greater  over  the  sea  than  over  the  land — possibly  due  to  the 
lack  of  oxidizable  matter;  and  this  may  explain  its  greater  pro- 


8  THE    ATMOSPHERE:     ITS   CONSTITUENTS 

portion  in  winter  than  in  summer.  The  proportion  is  greater 
on  clear,  dry  days  than  during  cloudy  spells.  The  daily  varia- 
tions of  the  ozone  content  of  the  atmosphere  seem  to  correspond 
to  the  variations  of  the  atmospheric  electric  potential. 

Dust  particles  so  fine  that  they  escape  measurement 
even  with  the  highest  power  of  the  microscope,  must  be  con- 
sidered a  part  of  the  normal  content  of  the  atmosphere.  Their 
presence  is  indicated  by  the  fact  that  they  may  reflect  enough 
light  to  make  them  visible  en  masse  when  a  powerful  light  is 
turned  upon  them  in  a  darkened  room,  or  when  a  searchlight 
throws  its  beam  at  night.  The  path  of  the  light  is  shown  by 
the  light  reflected  from  dust  motes.  Dust  particles  of  the  size 
thus  revealed  behave  like  molecular  rather  than  like  matter  of 
molar  sizes.  They  are  floating  matter,  the  particles  of  which 
may  not  settle  unless  they  are  brought  to  the  surface  by  means 
other  than  their  own  gravity. 

The  floating  dust  motes  of  the  air  are  factors  of  great  meteor- 
ological importance.  They  are  the  nuclei  upon  which  the  water 
vapor  of  the  air  condenses.  Dense  clouds  of  volcanic  dust  act 
as  a  screen  preventing  much  of  the  sun's  heat  from  reaching 
the  earth.  The  dust  particle  is  the  normal  nucleus  for  the 
cloud  particle.  The  flying,  or  windblown  dust,  though  a  highly 
important  physiographic  agent,  is  not  a  factor  of  importance  in 
meteorology. 

Chlorine  usually  occurs  in  the  air  of  localities  bordering 
upon  the  oceans,  and  sodium  chloride  reactions  may  be  obtained 
when  sea  winds  are  blowing  inland.  The  presence  of  the  salt 
is  due  to  the  action  of  the  wind  which  whips  a  small  amount 
of  spray  into  the  air.  The  chlorine  content  of  the  air  apparently^ 
plays  no  part  in  meteorology.  Like  smoke  and  chimney  products 
it  may  be  regarded  as  "foreign"  matter. 

From  the  foregoing  it  is  apparent  that  oxygen,  nitrogen, 
and  the  argon  group  of  gases  practically  constitute  the  "fixed" 
constituents  of  the  atmosphere.  Their  proportions  at  sea 
level  vary  but  little  in  different  parts  of  the  earth,  and  they 
constitute  about  98  per  cent  of  the  atmosphere.  Ozone,  the 
nitrogen  oxides,  ammonia  and  the  various  radio-active  emana- 
tions may  be  considered  practically  as  negligible  factors  in 
meteorology;  for  the  greater  part  they  are  accidental.  Carbon 
dioxide  is  a  factor  chiefly  in  physiological  meteorology. 


INFLUENCE  OF  WATER  VAPOR  9 

Water  vapor  and  the  unmeasured  dust  content  of  the  at- 
mosphere are  meteorological  factors  of  the  highest  degree  of 
importance.  All  the  fresh  waters  of  the  earth  are  derived  from 
the  sea  by  a  process  that  is  clearly  one  of  distillation;  and  life 
as  it  is  organized  on  the  earth  depends  upon  this  process.  Even 
a  slight  change  in  nature's  method  of  distillation  would  be 
followed  by  profound  changes  in  the  distribution  of  life. 

Indoor  Air  and  Mortality. — The  difference  between  the  sun- 
bathed air  of  out-of-doors  and  the  air  of  dwellings  has  exerted  a 
marked  effect  upon  modern  civilization.  Various  diseases  of  the 
densely  peopled  regions  of  Europe  and  America  are  practically 
unknown  among  peoples  who  live  habitually  out  of  doors. 
Tuberculosis  is  essentially  a  disease  of  modern  civilization.  Even 
in  Europe  and  America,  where  the  disease  thrives,  the  mortality 
is  twice  as  great  among  house  dwellers  as  among  those  having 
out-of-door  employment. 

The  mechanical  ventilation  of  buildings  has  helped  matters 
but  very  slightly.  Air  drawn  through  ventilating  shafts  has 
not  the  same  therapeutic  qualities  as  sunlit  air  coming  through 
open  windows  into  living  rooms.  An  explanation  of  the  differ- 
ence is  yet  to  be  found.  If  meteorology  is  the  science  of  the 
air,  it  should  discover  the  difference  between  wholesome  and 
unwholesome  air.1 

1  An  investigation  of  the  problem  has  been  undertaken  by  a  committee 
of  the  American  Meteorological  Society. 


CHAPTER  H 

FORMS  AND  PROPERTIES   OF  MATTER  IN  ITS 
RELATION  TO  METEOROLOGY 

Ether  and  Matter. — It  is  not  necessary  to  assume  that  the 
universe  and  space  are  one  and  the  same;  nor  that  the  universe 
is  boundless;  nor  that  space  is  without  limits.  So  far  as  that 
part  of  the  universe  with  which  human  knowledge  comes  in 
contact  is  concerned,  the  existence  of  two  factors  is  assumed. 
Matter  is  perceptible  to  the  human  senses.  It  is  visible,  tangi- 
ble, and  transformable.  It  can  be  measured  and  compared; 
some,  at  least,  of  its  properties  are  known.  Its  ultimate  consti- 
tution, however,  is  not  known.  It  is  usually  described  in  terms 
of  atoms,  molecules  and  masses. 

In  certain  respects,  more  is  known  about  ether  than  about 
matter:  for  although  the  existence  of  ether  is  merely  assumed,1 
the  magnitudes  attributed  to  it  are  real  values  that  have  been 
fully  established.  That  the  universe  is  pervaded  by  an  invisible, 
intangible,  but  measureable  something  is  conceded.  It  is 
assumed  that  the  manifestations  to  the  senses  known  as  heat, 
light,  magnetism,  electricity,  and  radiant  energy  traverse  the 
known  part  of  the  universe  by  the  means  of  the  ether.  It  is 
not  improbable  that  these  manifestations  are  undulations  of 
the  ether  itself. 

The  telescope  and  the  spectroscope  have  shown  that  the 
matter  entering  into  the  composition  of  other  visible  bodies  in 
the  universe  does  not  differ  from  that  which  composes  the 

l"In  recent  years,  doubt  as  to  the  necessity  for  assuming  the  existence 
of  an  ether  has  been  expressed  by  some  who  claim  that  it  is  sufficient  to  attribute 
the  power  of  transmitting  radiation  to  space  itself.  It  may  be  doubted  whether 
this  is  more  than  a  dispute  about  terms.  One  cannot  discuss  the  question, 
here;  but,  pending  the  settlement  of  the  controversy,  it  seems  wise  to  con- 
tinue the  use  of  the  word  'ether'  as  at  least  denoting  the  power  of  space, 
vacant  or  occupied  by  matter,  to  transmit  radiation." — DUFF,  A  Textbook 
of  Physics. 

10 


FORMS  OF  MATTER  M 

earth.  Many  of  the  chemical  elements  that  compose  the  earth 
have  been  discovered  in  the  sun  and  other  heavenly  bodies,  and 
no  chemical  element  has  been  discovered  in  any  heavenly  body 
that  does  not  occur  in  the  earth.  Air  and  water  vapor  occur  on 
the  planet  Mars,  and  it  is  not  unreasonable  to  assume  that  the 
meteorology  of  this  planet  has  much  in  common  with  the 
meteorology  of  the  earth.  The  occasional  occurrence  of  dust 
storms  on  Mars  adds  weight  to  the  reasonableness  of  such  an 
assumption. 

Forms  of  Matter. — For  practical  purposes  it  may  be  as- 
sumed that  matter  exists  in  three  forms — solid,  liquid  and 
gaseous.1  Most  of  the  metals  and  some  of  the  non-metals  may 
be  changed  easily  from  one  form  to  another.  Thus,  iron  is  a 
solid  at  ordinary  temperatures;  it  "melts"  or  liquefies  at  a 
temperature  somewhat  above  2100°  F  (1150°  C);  at  a  still  higher 
temperature  it  gives  off  a  reddish-brown  vapor.  Mercury  is 
ordinarily  a  liquid;  it  "boils"  or  becomes  a  vapor  at  675°  F 
(375°  Q  and  "freezes"  or  solidifies  at  -38°  F  (-39°  C).  Water 
is  the  most  common  illustration  of  all;  it  solidifies  at  32°  F 
(o°  C)  and  gradually  becomes  a  vapor  at  ordinary  temperatures; 
but  at  212°  F  (100°  C)  the  vapor  pressure  is  that  of  the  air  at 
sea  level.  Practically  all  the  ordinary  gases  have  been  liquefied 
and  solidified.  Liquid  air  and  carbon  dioxide  are  articles  of 
commerce. 

The  conditions  which  surround  the  liquefaction  of  ice  and 
snow,  the  evaporation  of  water,  and  the  condensation  of  the 
water  vapor  of  the  air  are  fundamental  factors  in  the  science 
of  weather.  The  distribution  of  precipitation — that  is,  rain, 
snow,  hail,  and  the  floating  forms  of  fog  and  cloud — affect  the 
habitability  of  the  earth  and  human  activities  to  a  very  great 
degree. 

Matter  may  be  changed  in  physical  form,  but  it  cannot  be 
annihilated.  Thus,  the  coal  in  the  fire-box  is  changed  to  carbon 
dioxide,  a  gas,  instead  of  a  solid;  but  the  chemist  may  separate 
the  carbon  from  the  oxygen.  Nothing,  not  even  the  energy,  is 
lost;  to  nature  nothing  can  be  added,  and  from  nature  nothing 
can  be  taken  away. 

Properties   of   Matter. — All   forms   of  matter  have   certain 

1  In  meteorology  the  discussion  of  the  radiant  form  of  matter  may  be 
omitted. 


12        FORMS  AND  PROPERTIES  OF  MATTER 

properties — volume,  density,  weight,  etc.,  in  common.  Other 
properties,  such  as  malleability  and  ductility,  affect  groups  or 
classes  of  matter — chiefly  the  metals. 

Cohesion  is  a  somewhat  archaic  term  denoting  molecular 
attraction.  In  solids,  the  cohesion  is  usually  strong,  so  that 
more  or  less  force  is  required  to  sunder  the  mass — that  is,  to 
separate  the  molecules.  The  resistance  of  cohesion  is  usually 
expressed  in  such  terms  as  tension,  torsion,  shearing,  etc.  In 
the  liquefaction  of  a  solid,  or  the  vaporization  of  a  liquid,  the 
force  employed  to  overcome  cohesion  is  measured  in  terms  of 
heat.  For  instance,  in  the  liquefaction  of  ice,  about  147  times 
as  much  heat  is  required  to  change  ice  at  32°  to  water  at  32°  as 
will  raise  the  temperature  of  the  same  weight  of  water  one 
degree  Fahrenheit  in  temperature.  In  the  case  of  liquids,  the 
cohesion  seems  to  be  slight,  inasmuch  as  the  molecules  possess 
a  considerable  mobility.  Nevertheless,  they  are  held  together 
by  a  powerful  force.  Thus,  the  heat  used  in  converting  one 
pound  of  water  at  212°  F  to  a  vapor  at  212°  F  would  raise  967 
pounds  of  water  one  degree  in  temperature.  Measured  thus,  in 
terms  of  heat  units,  great  power  is  required  to  overcome  molecu- 
lar attraction.  In  the  case  of  gases,  not  only  is  the  molecular 
attraction  negligible,  but  the  molecules  apparently  repel  one 
another.1  Perhaps  it  is  more  nearly  correct  to  say  that  they 
diffuse  themselves  throughout  the  space  which  contains  them. 
In  other  words  they  apparently  cease  to  possess  molecular 
attraction. 

Crystallization  is  a  form  of  molecular  attraction  which 
indicates  that  the  molecules  possess  a  certain  kind  of  polarity, 
usually  assuming  regular  geometric  forms.  Frost  and  snow- 
flakes  frequently  exhibit  marvelous  forms,  infinite  in  variety 
but  regular  and  similar  in  construction.  The  study  of  these 
forms  is  one  of  increasing  importance  in  weather  science. 

Expansion-contraction  is  a  property  of  matter  true  in  its 
ordinary  forms.  The  volume  of  a  substance  increases  when  it 
is  heated  and  contracts  with  cooling.  Thus,  iron  will  increase 

1  In  mechanics  the  repellent  force  of  the  water  vapor,  usually  called 
steam,  is  termed  "pressure"  and  is  rated  in  "pounds  per  square  inch,"  or 
in  "atmospheres"  of  14.7  pounds  per  square  inch.  In  meteorology  the 
repellent  force  is  expressed  sometimes  as  tension,  but  more  commonly  as 
Pressure. 


PROPERTIES  OF  GASES  13 

0.00000648  of  its  length  for  each  degree  F  of  increase  in  tem- 
perature, this  being  its  "coefficient  of  linear  expansion."  The 
coefficient  of  expansion  of  air  is  0.00367;  of  ethyl  alcohol,  0.0005; 
of  mercury,  0.0002. 1  In  meteorology  the  principles  of  this 
property  are  fundamental.  The  expansion  of  mercury  and  of 
alcohol  are  used  to  determine  the  intensity  of  heat;  and  to  the 
unequal  heating  of  the  air  in  different  localities  are  due  the 
movements  of  the  air — that  is,  the  winds. 

Magnetism  is  a  property  pertaining  chiefly  to  iron  and  steel, 
but  possessed  to  a  lesser  degree  by  other  metals.  When  in  the 
condition  known  as  magnetized,  a  piece  of  iron  or  of  steel 
attracts  and  holds  other  pieces  of  iron  and  steel.  Steel  retains 
its  magnetism  permanently;  iron  is  sensibly  magnetic  only 
when  within  magnetic  influence — that  is,  a  "magnetic  field." 
Nickel,  cobalt,  certain  manganese  alloys  and  tungsten  alloys 
exhibit  magnetism  very  sensibly.  A  bar  of  magnetized  steel, 
suspended  by  a  thread  attached  at  its  center  of  gravity,  comes 
to  rest  pointing  nearly  or  quite  north  and  south,  the  negative 
or  marked  end  pointing  in  a  general  way  to  the  earth's  north 
magnetic  pole.  A  few  substances,  chiefly  bismuth,  similarly 
suspended  come  to  rest  across  the  magnetic  field  when  be- 
tween the  poles  of  a  horseshoe  magnet.  The  investigations 
concerning  the  earth's  magnetic  properties  are  carried  on  in  the 
United  States  by  the  Coast  and  Geodetic  Survey. 

Properties  of  Gases. — Gases  are  perfectly  elastic.  A  gas 
fills  any  space  within  which  it  is  confined.  A  cubic  inch  of  a 
gas  whose  density  has  been  measured,  if  set  free  in  a  space 
whose  dimensions  are  a  cubic  foot,  or  a  cubic  yard,  will  fill  the 
space.2  Manifestly  its  density  and  tension  will  be  lessened  in 
proportion.  It  is  the  custom  to  say,  therefore,  that  a  gas  has 
no  specific  volume  of  its  own;  its  volume  is  that  of  the  con- 
tainer. 

Equal  volumes  of  a  gas,  temperature  and  density  remaining 
the  same,  contain  an  equal  number  of  molecules.  If  hydrogen, 
the  lightest  known  gas,  be  taken  as  the  unit  of  measurement, 

1  Different  values  are  given  by  different  authorities;    the  foregoing  are 
on  the  authority  of  H.  Whiting. 

2  This  property  of  gases  is  not  quite  true  at  temperatures  near  to  their 
condensation,  but  it  holds  good  at  temperatures  which  are  materially  higher 
than  the  temperature  approaching  condensation. 


14  FORMS  AND   PROPERTIES   OF   MATTER 

the  molecular  weight  of  any  gas  may  be  determined  by  compar- 
ing its  weight  with  that  of  an  equal  volume  of  hydrogen.  This 
is  known  as  Avogadro's  law. 

If  a  volume  of  gas  be  heated  from  32°  to  459°  F  1  (o°  to 
273°  C)  its  volume  will  be  doubled.1  That  is,  equal  volumes  of 
gases  expand  equally  with  the  same  increase  of  temperature. 

If  a  given  volume  of  gas  —  say  I  cubic  foot  of  oxygen  —  be 
introduced  within  a  container,  its  pressure  or  tension  noted, 
the  same  volume  of  another  gas  having  the  same  tension  may 
be  introduced  without  an  increase  of  tension  of  the  mixture. 
Thus  i  volume  of  oxygen  added  to  I  volume  of  nitrogen  will 
make  but  I  volume  of  the  mixture,  having  the  same  tension  as 
each  of  the  two  gases.  That  is,  one  gas  is  practically  a  vacuum 
for  another.  This  property  has  its  limitations;  when  several 
other  gases  are  introduced  within  the  container  a  noticeable 
increase  of  the  tension  of  the  mixture  takes  place. 

Inasmuch  as  the  science  of  meteorology  is  chiefly  a  study  of 
the  air,  a  mixture  of  gases  differing  in  their  specific  properties, 
a  clear  exposition  of  these  general  properties  is  necessary  to 
an  understanding  thereof  —  more  especially  to  the  solving  of  the 
problems  of  weather,  climate,  and  habitability. 

Gravity  is  a  property  of  matter  that  exists  apparently 
throughout  the  known  universe.  The  apparent  fact  that  matter 
in  masses  attracts  all  other  matter  in  masses  is  practically  all 
that  is  known  of  the  essence  of  it.  The  whirling  of  the  sun  and 
the  planets  about  a  common  center  of  gravity  balances  the  at- 
traction that  otherwise  would  bring  them  together.  More 
exactly  stated,  planetary  bodies  tend  to  move  in  straight  lines; 
gravity  tends  to  draw  them  to  a  common  center;  the  result  is 
orbital  movement.  These  complex  movements  and  forces  have 
a  great  and  very  measureable  influence  on  the  movements  of 
the  sea  and  the  air. 

It  is  convenient  to  note  the  density  —  practically  the  "weight" 

1  This  may  be  expressed  by  the  formula 


where  V  is  the  given  volume;  V  the  volume  sought;  /  the  given  temperature; 
/'  the  temperature  of  the  volume  sought;  and  k,  0.00367.  This  is  known 
as  Boyle's  law,  and  also  as  Mariotte's  law.  It  is  true  at  high  temperatures, 
but  not  exact  at  ordinary  temperatures. 


GRAVITY  15 

— of  various  kinds  of  matter,  comparing  them  volume  for 
volume,  under  standard  conditions,  with  the  density  of  a  given 
substance.  This  ratio  of  weight  is  the  specific  gravity  of  the 
substance.  Distilled  water  at  its  maximum  density,  39.1°  F 
(3.94°  C)  is  usually  taken  for  the  comparison.  Thus,  a  given 
volume  of  mercury  weighs  13.6  times  as  much  as  an  equal 
volume  of  water,  and  an  equal  volume  of  alcohol  0.81  times  as 
much  as  an  equal  volume  of  water. 

For  gases,  air  and  hydrogen  are  both  used  as  units  of  com- 
parison. The  specific  gravity  of  hydrogen,  in  terms  of  air,  is 
0.069;  °f  air  m  terms  of  hydrogen,  14.4;  of  coal  gas,  commonly 
used  for  inflating  balloons,  about  0.061 ;  of  water  vapor,  0.62. 
When  metric  units  are  employed,  the  weight  of  a  cubic  deci- 
meter is  the  specific  gravity  of  the  given  substance,1  a  cubic 
decimeter  of  water  under  standard  conditions,  weighing  in 
theory,  but  not  in  fact,  I  kilogram. 

wt 

1  The  following  formulas  are  useful:   Sp.  gr.  = ;   weight  =  vol. Xsp.gr. ; 

vol. 

wt. 

volume  = . 

sp.  gr. 


CHAPTER  III 
HEAT:    ITS  NATURE,  PROPERTIES  AND  DIFFUSION 

The  Nature  of  Heat. — The  phenomena  of  heat  and  light  are 
described,  the  one  as  "molecular  motion,"  the  other  as  the 
"radiation  of  solar  energy."  Roughly,  either  definition  will 
apply  to  either  phenomenon.  Not  much  is  known  about  the 
real  essence  of  either,  except  that  they  are  forms  of  energy 
which  have  been  measured,  and  of  which  certain  magnitudes 
have  been  established  under  the  name  of  "wave  lengths." 

Radiant  Heat. — It  is  assumed  that  heat  and  light  traverse 
space  in  "waves"  or  vibrations  of  the  ether.  Positive  knowledge 
is  confined  to  the  fact  that  heat  is  radiated  by  the  sun  and 
stars  in  every  direction.  Practically  all  the  heat  received  by 
the  earth  comes  from  the  sun;  and  of  the  whole  amount  radi- 
ated by  the  sun,  the  earth  intercepts  less  than  one  two-billionth 
part.  Nevertheless,  this  small  fraction  of  the  sun's  radiant  heat 
produces  all  the  results  upon  the  earth  which  are  manifested 
by  life  and  its  activities. 

Some  of  the  ether  waves  stimulate  the  nerves  of  the  eye, 
producing  the  phenomena  of  light  and  vision.  Others  do  not 
affect  the  nerves  of  sight;  as  they  fall  on  the  body  they  produce 
the  sensation  of  warmth,  thereby  stimulating  the  growth  of 
living  matter.  Meteorology  is  concerned  chiefly  with  radiant 
energy  of  this  character;  they  are  conveniently  called  heat  waves. 

Perhaps  our  nearest  approach  to  actual  knowledge  of  heat  is 
the  recognition  of  the  fact  that  when  heat  waves  fall  upon 
matter — say,  a  piece  of  metal — they  set  up  a  motion  in  the 
molecules  composing  it.  If  the  intensity  of  the  waves  increases, 
molecular  attraction  little  by  little  is  overcome;  the  solid  be- 
comes a  liquid  and,  finally,  a  vapor.  This  important  change  is 
explained  as  being  due  to  increasing  and  to  wider  amplitude  of 
the  oscillations  of  the  molecules.  Perhaps  the  theory  may  not 
be  satisfactorily  established,  but  the  facts  cannot  be  denied; 

16 


SOURCES  OF  HEAT  17 

the  heat  has  increased  the  motion  of  the  molecules;  finally  it 
has  overcome  their  cohesion. 

Sources  of  Heat. — The  warmth  that  is  concerned  with  life 
and  its  activities  is  derived  from  the  sun.  The  sun  warms  the 
rock  envelope  of  the  earth;  the  rock  envelope  radiates  warmth 
to  the  atmosphere;  the  movements  of  the  air  diffuse  the  warmth, 
bringing  cool  air  into  warm  regions  and  sending  warm  air  into 
cold  regions.  Weather  science  is  concerned  chiefly  with  these 
movements  of  the  air. 

The  earth  itself  is  a  source  of  heat.  The  interior  of  the  rock 
envelope  of  the  earth  is  intensely  hot.  Borings  into  the  rock 
envelope  show  an  increase  of  temperature  with  depth.  The 
rate  varies  with  the  character  of  the  rock,  a  rough  average  being 
i°  F  for  every  70  feet.1  Some  of  the  heat  of  the  rock  envelope, 
at  such  depths,  is  due  to  chemical  action  going  on  within  the 
rocks  themselves;  some  is  due  to  vulcanism;  but  some  is  cer- 
tainly due  to  the  radiation  of  the  heat  of  the  interior  of  the  rock 
envelope  itself.  In  meteorology  this  source  of  heat  practically  is 
negligible. 

The  most  common  example  of  a  terrestrial  source  of  heat 
is  the  ordinary  combustion  of  fuels.  But  even  this  is  apparent 
rather  than  real  terrestrial  heat;  in  fact  it  is  the  heat  of  the  sun 
stored  and  conserved  by  vital  chemical  processes.  But  all  the 
original  heat  derived  from  within  the  earth  and  all  the  heat 
of  vital  chemical  processes  bears  an  infinitesimal  ratio  to  that 
received  from  the  sun.  It  is  estimated  that  the  heat  received 
by  the  earth  in  one  minute  is  sufficient  to  raise  42,000,000,000 
tons  of  water  from  the  freezing  to  the  boiling  point. 

Heat  as  Motion. — If  heat  produces  molecular  motion,  it 
must  be  assumed  that  the  ether  waves  which  traverse  space 
warm  nothing  until  they  fall  on  matter — that  is,  on  a  substance 
composed  of  molecules  which  can  be  set  in  motion.  But  in 
many  respects  heat  behaves  to  matter  much  as  light  does.  It 
may  pass  through  a  substance  just  as  light  passes  through  glass; 
such  a  substance  is  said  to  be  diathermous .  Thus,  glass  itself  is 
more  or  less  diathermous,  so  also  is  clear  water.  Glass  permits 
most  of  the  heat  of  the  sun  to  pass  through  it,  but  it  intercepts 

1  At  the  Goff  well,  near  Bridgeport,  West  Virginia,  the  temperature  at  a 
depth  of  7310  feet  is  159  F  (106°  C).  The  average  increase  of  many  measure- 
ments is  somewhat  less  than  i°  for  each  70  feet. 


18        HEAT:    ITS  NATURE,  PROPERTIES  AND  DIFFUSION 

some  of  it.  Thereby  molecular  motion  is  set  up  in  the  glass 
itself  and  it  becomes  warm,  radiating  heat  in  just  the  same 
manner  as  a  stove  radiates  it. 

The  heat  "absorbed"  by  a  substance — that  is,  converted 
into  molecular  motion — in  time  may  be  stored  until  it  is  again 
radiated  and  is  given  out  as  warmth.  It  is  conveniently  called 
"sensible  heat."  It  is  thus  distinguished  from  the  ether  waves 


After  Ferrel. 
Convectional  movements  of  the  air;  sectional  view. 

that  have  great  physical  power,  but  do  not  directly  impart  the 
sense  of  warmth.  One  cannot  draw  a  line  between  the  sensible 
and  the  ultra-sensible  heat  waves,  however;  and  the  use  of  the 
term,  though  convenient,  is  not  exact.  Weather  science  deals 
chiefly  with  the  sensible  heat  of  the  air. 

Diffusion  of  Heat. — As  a  body  becomes  warm,  the  heat  may 
diffuse  itself  through  the  mass  rapidly,  as  in  the  case  of  metals, 


CONVECTION  OF  THE  AIR  19 

or  slowly,  as  in  the  case  of  non-metals.  The  former  are  good 
conductors;  the  latter  are  sometimes  regarded  as  poor  con- 
ductors, or  insulators.  Thus,  steam  pipes  are  wrapped  with 
asbestos  coverings.  The  metal  pipe  itself  warms  rapidly  and, 
radiating  heat  rapidly,  causes  a  loss  of  heat  in  the  steam.  The 
asbestos  covering,  being  a  non-conductor,  or  insulator,  prevents 
the  loss  by  radiation.  In  solid  bodies,  the  diffusion  of  heat  is 
accomplished  by  conduction.  The  motion  imparted  to  molecules 
sets  the  molecules  nearest  to  them  in  motion;  finally  the  heat 
is  diffused  throughout  the  mass. 

When  liquids  and  gases  are  heated  a  movement  of  mixing 
occurs.  This  process  is  best  observed  when  a  handful  of  saw- 
dust is  placed  in  a  beaker  of  water  and  the  water  is  heated  by 
a  Bunsen  burner.  The  rapid  warming  of  the  water  carries  the 
particles  of  sawdust  upward,  outward  and  downward  through 
the  water;  they  indicate  the  progressive  movement  of  the 
water  in  different  parts  of  the  beaker.  This  mixing  process  is 
called  convection. 

Convection  of  the  Air. — The  convectional  movements  of  the 
air  are  among  the  most  important  factors  in  weather  science. 
Aside  from  the  lateral  movements  of  the  air — the  winds — con- 
vectional movements  that  are  more  or  less  vertical  are  going  on 
all  the  time;  that  is,  air  is  going  up  or  coming  down.1  Warm 
air  is  ascending,  cool  air  is  descending. 

The  sun  does  not  warm  all  parts  of  the  earth  evenly.  In 
tropical  latitudes  the  heat  is  far  more  intense  than  in  extra- 
tropical  latitudes.  Because  of  the  curvature  of  the  earth's 
surface,  polar  regions  receive  the  sun's  rays  very  obliquely. 
The  unequal  heating  results  in  a  convectional  movement  of  the 
air  on  a  scale  that  affects  the  whole  atmosphere.  It  produces  an 
upward  and  poleward  flow  of  air  in  tropical  regions  which  is 
balanced  by  a  downward  and  tropic-ward  movement  of  the  air 
in  extra-tropical  regions. 

The  principle  of  convection  is  one  of  the  most  important 
in  meteorology,  and  is  practically  the  foundation  of  that 

XW.  J.  Humphreys  points  out  the  interesting  paradox  that  "more  air 
goes  up  than  comes  down."  Ascending  air  carries  water  vapor,  an  integral 
part  of  the  air.  But  the  updraught  chills  and  condenses  the  water  vapor 
which  falls  as  rain  or  as  snow.  The  descending  air  is  less  in  quantity  by 
the  amount  of  water  vapor  that  is  lost  by  condensation. 


20        HEAT:    ITS  NATURE,  PROPERTIES  AND  DIFFUSION 

part  of  weather  forecasts  which  concerns  storms  and  cold 
waves. 

Specific  Heat. — Different  substances  vary  greatly  in  their 
"capacity"  for  heat.  That  is  a  much  greater  amount  of  heat  is 
required  to  produce  a  given  intensity  of  molecular  motion 
in  one  kind  of  matter  than  in  another.  For  convenience,  the 
amount  is  called  the  thermal  capacity  of  the  substance.  For 
convenience  also,  the  heat  taken  up  by  a  given  weight  of  water 
is  taken  as  the  unit  of  measurement.  Thus,  a  pound  of  water 
has  9  times  the  thermal  capacity  of  the  same  weight  of  iron  and 
30  times  that  of  mercury.1 

Latent  Heat. — Reference  has  already  been  made  to  the 
fact  that  a  very  great  amount  of  heat  disappears  when  water 
at  212°  F  (100°  C)  is  converted  to  steam  at  212°  F.  The  heat 
apparently  lost  reappears  when  the  steam  is  condensed  to  water 
at  212°  F.  The  heat  thus  employed  in  overcoming  molecular 
attraction  is  called  latent  heat.  The  latent  heat  of  evaporation 
is  an  important  factor  in  the  diffusion  of  heat.  Thus,  water 
vapor  from  tropical  regions  is  borne  to  higher  latitudes  and 
there  condensed,  setting  free  an  enormous  amount  of  latent 
heat,  which  becomes  "sensible"  heat  again.  The  latent  heat 
set  free  when  water  freezes  is  also  a  factor  in  climate. 

Adiabatic  Heating  and  Cooling. — If  a  volume  of  gas,  or  of 
air,  is  compressed,  a  noticeable  amount  of  heat  is  given  off. 
The  hand-operated  tire  pump  is  an  example;  after  a  dozen 
strokes  of  the  plunger  the  barrel  of  the  pump  becomes  hot.  If 
the  compressed  air  expands  to  its  original  volume,  just  as  much 
heat  is  absorbed  in  the  expansion  as  was  given  off  during  com- 
pression. The  ordinary  ammonia  gas  compressor  furnishes  an 
instructive  illustration.  The  pipe  near  the  compression  valve 
may  be  at  a  low  red  heat;  the  pipe  at  the  release  valve  is 
usually  cased  with  a  thick  jacket  of  ice.  Heat  has  not  been 
added  to  the  gas  in  the  process  of  compression ;  it  has  not  been 
taken  away  during  expansion, 

1  Thus,  if  the  specific  heat  of  water  is  I,  that  of  iron  is  0.1138;  of  mercmy 
0.0333;  of  glass,  0.1977;  of  dry  air  at  constant  pressure,  0.2375;  of  steam 
at  212°  F,  0.341;  of  ice,  0.50.  Because  of  its  great  specific  heat  and  good 
conductivity  it  is  evident  that  water  is  well  adapted  to  the  heating  of 
buildings.  It  holds  its  warmth  steadily;  it  also  holds  a  greater  amount  than 
any  other  available  substance. 


UNITS  OF  MEASUREMENT  21 

This  phenomenon  is  an  important  principle  of  weather 
science.  As  has  been  pointed  out,  convection  in  the  air  is  always 
going  on.  Ascending  air  expands  in  volume  because  of  de- 
creasing pressure;  descending  air  is  compressed  in  volume 
because  of  increasing  pressure.  Therefore  it  follows  that  ascend- 
ing air  cools  by  expansion  and  descending  air  becomes  warmer 
by  compression.  This  phenomenon  is  called  adiabatic  heating 
and  cooling;  to  all  intents  and  purposes  it  is  merely  a  form  of 
latent  heat.  Adiabatic  heating  and  cooling  of  the  air  therefore 
is  practically  due  to  convectional  movements. 

Units  of  Measurement. — Various  units  are  employed  in 
the  measurement  of  heat.  Two  aspects  of  heat  measurement 
concern  meteorology — quantity  and  intensity.  Thus,  the  quan- 
tity of  heat  which  a  given  weight  of  a  substance  may  contain  is 
less  than  that  of  a  greater  weight  of  the  same  substance,  and, 
as  has  been  noted,  equal  weights  of  different  substances  may 
differ  greatly  in  thermal  capacity.  Several  units  of  quantity, 
or  thermal  capacity,  are  employed.  The  calorie  is  the  amount 
of  heat  required  to  raise  one  gram  of  pure  water  one  degree 
centigrade  in  temperature.  This  unit  is  employed  very  generally 
in  scientific  research.  In  some  instances,  however,  it  is  more 
convenient  to  employ  the  great  calorie,  or  the  amount  of  heat 
required  to  raise  one  kilogram  of  water  one  degree  centigrade 
in  temperature.  The  British  thermal  unit,  the  heat  required  to 
raise  one  pound  of  water  one  degree  Fahrenheit,  is  also  much 
used — chiefly,  however,  in  expressing  the  heat  value  of  fuels. 

The  unit  of  intensity  is  the  degree,  of  which  there  are  several, 
each  differing  in  value  from  the  others.  All  of  them,  however, 
have  a  common  basis — namely,  the  difference  in  intensity  of 
molecular  motion  between  melting  ice  and  boiling  water,  under 
certain  standard  conditions.  The  various  scales  of  degrees  are 
explained  in  another  chapter. 

The  Solar  Constant. — Weather  science  is  concerned  in  the 
amount  of  heat  received  by  the  earth  from  the  sun.  For  express- 
ing this  value  the  calorie  is  used.  The  measurements  begun  by 
Angstrom  and  Langley,  and  continued  by  Abbott,  Kimball 
and  others,  cover  a  period  of  about  forty  years.  Simultaneous 
cooperative  observations  carried  on  in  the  United  States  and 
elsewhere  show  that  the  value  is  by  no  means  constant,  but  that 
it  varies  from  time  to  time.  The  mean  of  observations  deduced 


22       HEAT:    ITS  NATURE,  PROPERTIES  AND  DIFFUSION 

by  Abbott  is  1.932  calories  per  minute,  less  the  amount  ab- 
sorbed by  the  air. 

The  air,  with  its  dust  and  its  moisture  content,  intercepts 
a  great  deal  of  the  heat  radiated  from  the  sun.  When  the  sky 
is  clear  and  the  sun  is  overhead,  it  is  found  that  a  little  more 
than  two-thirds  of  the  sun's  radiation  reaches  the  earth,  less 
than  one-third  being  absorbed  by  the  atmosphere.  When  the 
moisture  content  of  the  air  increases,  the  value  of  the  solar 
constant  decreases.  When  the  smoke  pall  that  hovers  over 
manufacturing  centers  thickens,  the  effect  is  the  same.  This 
also  is  true  of  any  increase  of  atmospheric  dust.  The  volcanic 
dust  shot  into  the  air  by  the  eruption  of  Krakatoa  lowered  the 
value  of  the  solar  constant  for  a  considerable  length  of  time. 

The  moisture  and  dust  content  of  the  air  acts  as  a  blanket, 
intercepting  and  storing  during  the  day  a  part  of  sun's  heat, 
and  at  night  becoming  a  source  of  heat  in  itself. 

The  fixed  constituents  of  the  air,  the  oxygen  and  the  nitrogen, 
vary  so  slightly  in  proportion  and  the  amount  of  heat  which 
they  intercept,  that  their  effects  may  be  regarded  as  constant. 
The  great  changes  in  the  effects  of  insolation  are  due  chiefly  to 
the  varying  proportions  of  the  water  vapor  and  the  dust  content 
of  the  air.  The  layer  of  water  vapor  is  comparatively  thin — 
practically  not  more  than  five  or  six  miles.  The  dust  blanket, 
on  the  other  hand,  may  extend  many  miles  into  the  upper  air. 


CHAPTER   IV 


THE  AIR;    THE  DISTRIBUTION   OF  WARMTH 

Disregarding  the  very  slight  amount  of  heat  radiated  from 
the  earth's  interior  to  the  surface,  and  also  that  received  from 
other  heavenly  bodies,  the  sun  must  be  regarded  as  the  source 
of  the  heat  received  at  the  earth's  surface.  The  greatest  in- 
tensity of  heat  is  received  in  equatorial  regions  where  the  sun's 
rays  are  practically  vertical;  the  least  intensity  is  in  polar 
regions  where  the  rays  fall  obliquely. 

The  inclination  of  the  earth's  axis  to  the  plane  of  the 
ecliptic,  23°  27',  is  an  im- 
portant factor  in  the  distribu- 
tion of  warmth.  The  direction 
of  the  axis,  minor  oscillations 
excepted,  is  constant;  it  ranges 
very  nearly  toward  the  north 
star.  One  result  of  the  con- 
stant parallelism  of  the  earth's 
axis  is  a  movement  of  the  belt 
of  vertical  rays — the  "  heat 
belt" — back  and  forth,  an 
angular  distance  of  nearly  47 
degrees — from  the  Tropic  of 
Cancer  to  the  Tropic  of  Cap- 


Redway's  Physical  Geography. 
Relative  length  of  day  and  night. 


ricorn.  The  polar  circles, 
23°  27'  from  each  pole,  mark 
the  farthest  point  beyond 

each  pole  to  which  the  sun's  rays  extend  when  vertical 
at  a  tropic.  Another  result  of  the  inclination  of  the  earth's 
axis  is  the  increasing  length  of  summer  days  and  winter  nights 
as  the  latitude  increases.  At  either  tropical  circle  the  longest 
day  is  a  little  more  than  13.5  hours;  the  shortest,  about  10.5 
hours.  Within  the  temperate  zones  the  longest  days  vary  from 

23 


24  THE   AIR:    THE    DISTRIBUTION   OF   WARMTH 

13.5  hours  to  24  hours.  The  possible  hours  of  daily  sunshine 
vary  according  to  month  and  according  to  latitude. 

Polar  and  tropical  circles  are  the  boundaries,  not  of  climatic, 
but  of  light  zones.  The  duration  of  daylight  is  of  great  im- 
portance; it  governs,  in  no  small  degree,  the  maturing  of  crops, 
and  therefore  concerns  practically  all  agricultural  industries. 
In  general,  the  regions  of  greatest  productivity  of  staple  food- 
stuffs are  those  in  which  the  summer  days  are  from  14  hours  to 
1 6  hours  long.  Both  the  navigator  and  the  aviator  must  know 
whether  he  heads  in  the  direction  of  increasing  or  of  decreasing 
hours  of  daylight  at  any  particular  time  of  the  year. 

Climatic  Zones. — Climatic  zones  correspond  pretty  closely 
to  light  zones,  in  position;  but  their  boundaries  are  very  ir- 
regular lines,  called  isothermal  lines — that  is,  lines  along  which 
the  annual  mean  temperature  is  the  same.  For  all  practical 
purposes,  the  climatic  torrid  zone  is  the  zone  where  frost  does 
not  occur  except  at  very  high  altitudes.  Similarly,  the  southern- 
most line  at  which  frost  may  occur  is  the  southern  boundary 
of  the  north  temperate  zone;  and  the  line  of  mean  temperature 
of  32°  (o°  C)  may  be  considered  its  northern  boundary.  A 
more  practical  boundary  is  sometimes  fixed  at  the  northern 
limit  at  which  barley  will  mature. 

Climatology  is  chiefly  concerned  with  the  regions  which 
will  produce  foodstuffs,  and  therefore  sustain  life.  To  a  lesser 
degree  it  is  concerned  with  the  problems  which  affect  transporta- 
tion. In  any  case  the  problems  are  mainly  those  of  temperature, 
pressure,  moisture,  wind  and  sunshine. 

The  Diffusion  of  Warmth. — The  warmth  of  the  various  parts 
of  the  earth  is  modified  chiefly  by  the  movements  of  the  air. 
Because  of  the  vertical  rays  of  the  sun  in  equatorial  regions, 
the  air  is  not  only  warmed  to  a  much  higher  temperature,  but 
it  is  also  warmed  more  quickly  than  in  higher  latitudes.  Being 
expanded  by  the  greater  warmth,  it  becomes  specifically  lighter 
and  is  pushed  upward  by  the  denser  cold  air  which  flows  in  to 
take  its  place.  The  updraught  of  air  flows  poleward  in  upper 
currents,  until  it  is  chilled  and  descends  to  the  surface  again. 
A.  part  of  the  descending  current  continues  poleward  but  a  con- 
siderable part  flows  back  to  tropical  regions  as  a  surface  wind.1 

1  This  explanation  is  not  accepted  by  all  meteorologists,  but  it  is  supported 
by  evidence  that  cannot  be  disregarded. 


DIFFUSION  OF  WARMTH 


25 


The  actual  movements  of  convection  are  much  more  complex. 
Calms  alternate  with  eddying  movements  of  great  intensity. 
All  general  movements  are  deflected  by  the  rotation  of  the 
earth  on  its  axis — easterly  in  tropical  latitudes,  and  westerly 
beyond  the  tropics.  There  are  therefore  three  wind  belts, 
one  of  easterly  and  two  of  westerly  motion.  Each  of  these  has 
also  a  northerly  and  a  southerly  component;  moreover,  all 
three  belts  shift  alternately  north  and  south  with  the  apparent 
movement  of  the  sun.  The  belt  of  tropical  easterly,  or  Trade 
Winds,  extends  a  little  further  north  than  New  Orleans  in  sum- 
mer and  its  northern  edge  recedes  as  far  south  as  Havana  in 
winter.  The  position  of  the  wind  belts,  month  by  month,  is 


June 


January 


The  migration  of  the  heat  belt. 


shown  on  the  Coast  Pilot  charts  of  the  United  States  Hydro- 
graphic  Office. 

The  apparent  motion  of  the  sun,  due  to  the  inclination  of 
the  earth's  axis,  carries  the  zone  of  greatest  warmth  far  north 
in  June  and  far  south  in  December.  Thereby  the  warmth  of 
tropical  regions  is  carried  well  into  the  temperate  zones,  and 
thereby  the  production  of  foodstuffs  is  extended  to  about  the 
sixtieth  parallel  of  latitude,  north  and  south. 

All  this  complexity  of  movement  adds  to  the  diffusion  of 
warmth.  The  warm  air  of  tropical  regions  is  mixed  with  the 
cold  air  of  circumpolar  regions.  Complex  as  they  are,  the 
general  movements  of  diffusion  may  be  classified  as  the  hori- 
zontal movements  which  include  the  winds,  and  the  vertical 


26  THE   AIR:     THE    DISTRIBUTION    OF    WARMTH 

con vectional  movements  with  which  are  classed  the  cyclones  and 
the  anticyclones. 

Temperature  and  Altitude. — The  effects  of  altitude  on  tem- 
perature may  be  considered  in  two  aspects — altitude  along  a 
sloping  surface,  such  as  that  of  a  mountain  range,  or  a  high 
plateau,  and  altitude  above  the  surface,  directly  into  the  air. 
Altitudes  are  measured  usually  from  mean  sea  level. 

The  variations  in  temperature  of  the  various  plains,  plateaus, 
and  mountain  ranges  are  very  great.  In  general,  the  tempera- 
ture decreases  with  altitude  until,  in  tropical  regions,  the  limit 
of  perpetual  snow  is  reached  at  a  height  of  about  16,000  feet; 
it  decreases  with  increase  of  latitude  until,  in  circumpolar 
regions,  the  snow  line  is  not  much  above  sea  level.  The  varia- 
tions of  temperature  with  height  are  governed  by  so  many 
conditions  that  specific  rules  apply  to  specific  localities  only. 

The  study  of  the  relations  between  temperature  and  vertical 
altitudes  is  a  matter  of  great  importance  in  meteorology,  and 
it  has  been  prosecuted  diligently  during  the  last  quarter  of  a 
century  in  various  parts  of  the  United  States,  Canada,  Europe, 
South  America  and  Africa. 

Many  thousand  flights  have  been  made  by  kites,  manned 
balloons,  captive  balloons,  pilot  balloons,  sounding  balloons, 
airplanes  and  dirigible  airships.  At  Uccle,  Belgium,  a  pilot 
balloon  reached  an 'altitude  of  20.1  miles,  or  32,430  meters. 
Up  to  an  altitude  of  about  9  miles,  temperature  and  pressure 
statistics  of  the  air  have  been  obtained  for  about  every  thousand 
feet  of  altitude;  beyond  that  plane  the  measurements  are  in- 
complete. 

The  fall  in  temperature  with  the  increase  in  altitude  has 
been  in  the  traditional  ratio  of  i°  F  for  every  300  feet1 — the 
conventional  temperature  gradient.  This  has  been  a  convenient 
ratio  for  general  purposes,  but  it  cannot  be  used  in  specific 
cases.  Within  the  first  2  miles  the  temperature  gradient  is 
very  irregular;  at  times  there  is  even  a  rise  in  temperature  with 
increased  altitude;  that  is,  the  temperature  gradient  becomes 
negative.  The  rise  in  temperature  with  increasing  altitude  is 
technically  known  as  inversion. 

Inversion  may  occur  in  winter,  when  comparatively  still  air 

1  This  does  not  refer  to  the  adiabatic  cooling  of  air  by  expansion — about 
i°  F  per  183  feet,  or  i°  C  per  100  meters. 


TEMPERATURE  AND  ALTITUDE 


27 


Altitude 
KUometew, 


Fahrenheit 


-60°-58J-4(r-22J-4°    14°  32°  50°  68      ^- 


settles  on  a  level  surface  or  in  a  basin.  It  is  pretty  apt  to  be 
noticeable  when  a  cloud  layer  separates  two  layers  of  air;  the 
upper  layer  may  be  the  warmer ; 
indeed,  the  airman  is  quite  apt 
to  find  a  higher  temperature 
above  than  below.  Above  a 
height  of  2  miles,  when  the  air 
is  moderately  still,  the  fall  in 
temperature  is  apt  to  be  fairly 
uniform.  At  a  height  varying 
from  nearly  7  to  10  miles  the  fall 
in  temperature  ceases.  Above 
this  plane  it  remains  stationary, 
or  perhaps  it  rises.  In  one  in- 
stance, a  steady  rise  of  tempera- 
ture was  observed  between  the 
altitudes  of  8  miles  and  20 
miles. 

The  plane  which  separates 
the  stratum  of  falling  tempera- 
ture from  that  of  stationary 
temperature  is  sometimes,  but 
rather  loosely,  called  the  isother- 
mal layer.  It  varies  in  height, 
being  highest  at  the  equator;  it 
is  likewise  higher  in  summer  than 
in  winter.  It  separates  the  shell 
of  the  atmosphere  into  two  dis- 
tinct layers — the  stratosphere, 
and  the  troposphere. 

The  air  of  the  stratosphere  is 
remarkable  chiefly  for  its  appar- 
ent inertness.  At  its  lower  part 
the  temperature  does  not  vary 
much  from  -67°  F  (-55°  C). 
If,  as  seems  probable,  there  is  a 
rise  of  temperature  with  increase 
of  altitude,  the  rise  is  normal 
rather  than  abnormal.  It  seems 


^60J-  5(r-40°-  30 -20  "-10°  0°    10°  20°  30> 
Centigrade 

Temperature  records  made  by  a 
sounding  balloon  at  Avalon,  Califor- 
nia, July,  1913.  Note  that  an  inver- 
sion of  temperature  occurs  at  the 
altitude  of  about  12  miles,  and  at  20 
miles  the  temperature  is  about  20 
degrees  higher  than  at  12  miles. 


to  be  due  to  the  fact  that  the  base  of  the  stratosphere  is  chilled 


28  THE   AIR:     THE    DISTRIBUTION    OF    WARMTH 

by  masses  of  extremely  cold  air  that  constantly  are  thrown 
upward  against  it. 

The  humidity  of  the  air  of  the  stratosphere  is  very  low — so 
low  that  visible  clouds  do  not  form.  Therefore,  if  the  dew- 
point  is  ever  reached,  the  condensation  is  confined  to  ice  spicules 
so  few  in  number  that  they  do  not  affect  the  visibility  of  the 
air.  There  is  no  vertical  convection ;  therefore  they  sink  slowly; 
and  if  they  are  greater  in  size  than  are  molecules  of  water 
vapor  they  sink  more  rapidly  than  the  water  vapor  diffuses  itself. 

It  seems  certain  that  the  air  of  the  stratosphere  contains 
dust  a-plenty — both  cosmic  dust  and  dust  that  is  hurled  into  it 
by  volcanic  eruptions.  If  dust  is  absent,  the  air  of  the  strato- 
sphere differs  from  that  below  it  and  from  space  above  it.  One 
thing  is  certain,  the  radio-activity  within  the  stratosphere  in- 
dicates the  presence  of  dust  particles  highly  electrified. 

The  depth  of  the  troposphere  is  inconsiderable  compared 
•with  that  of  the  stratosphere;  aviation  has  probably  scaled  its 
height  probably  within  pistol  shot  distance  of  the  isothermal 
layer.  The  troposphere  is  the  region  of  convection.  Its  height 
is  practically  the  height  of  cirrus  clouds,  and  all  the  great  move- 
ments of  the  air — wind,  cloud,  storm,  and  precipitation — take 
place  within  its  limits. 

Experience  has  taught  the  meteorologist  that  conditions  in 
mid-air  of  the  troposphere,  in  many  instances,  are  the  key  to 
conditions  at  the  surface.  They  are  far  more  important  in  air 
flight;  for  the  airman  encounters  bumps  and  holes,  both  of  which 
are  due  to  sudden  inequalities  in  temperature.  The  airman 
and  the  navigating  officer  of  the  airship  are  likely  to  encounter 
cross-winds,  the  updraught  of  thunder-storms,  and  the  vagaries 
of  cloud-formation;  these,  too,  are  due  to  irregular  conditions 
of  temperature,  all  of  which  must  be  understood  and  reckoned 
with  in  flight. 

Air  Altitudes  and  Terrain  Altitudes. — The  laws  and  values 
which  apply  to  vertical  altitudes  in  free  air  are  not  applicable 
to  altitudes  on  the  earth's  surface.  In  general,  temperature  de- 
creases with  altitude,  but  this  is  not  always  true.  At  various 
times  the  temperature  of  mountain  valley  floors  is  lower  than 
that  of  the  foot-hill  slopes  several  hundred  feet  higher.  On 
very  cold,  still  nights,  low  spots,  such  as  stream  valleys,  are 
almost  always  colder  than  higher  ground.  In  regions  where 


TEMPERATURE  AND  LATITUDE  29 

late  frosts  prevail,  fruit  growers  have  learned  to  take  advantage 
of  this  fact.  The  difference  between  the  temperature  of  a  low 
spot  and  higher  ground  a  few  rods  away  is  at  times  the  difference 
between  freezing  and  non-freezing  temperature. 

In  tropical  regions  where  mountains  lie  against  the  coast 
the  difference  between  sea  level  temperature  and  that  of  the 
foot-hills  a  thousand  feet  higher  is  very  marked.  Thus,  the 
temperature  of  the  business  district  of  Victoria,  Hongkong,  is 
almost  intolerable  to  Europeans;  on  the  Peak,  a  few  hundred 
feet  higher,  the  climate  is  pleasant.  The  same  difference  is 
noticeable  between  Rio  Janeiro  and  its  suburb,  the  Corcovado; 
it  is  even  more  noticeable  in  comparing  the  climate  of  Vera 
Cruz  with  that  of  Puebla  or  Orizaba. 

On  the  other  hand,  extremely  hot  days  in  the  foot-hills  of  the 
Sierra  Nevada  Mountains  are  apt  to  be  cool  days  along  the  coast. 
The  explanation  is  not  hard  to  find:  the  ascending  hot  air  of 
the  foot-hills  is  replaced  by  cold  air  blowing  in  from  the  ocean. 

In  many  instances  the  difference  between  low  valley  and  hill 
stations  is  quite  as  much  hygienic  as  climatic.  It  is  the  dif- 
ference between  moist,  dusty  and  miasmatic  air  on  the  one 
hand;  and  clear,  dry  air  on  the  other. 

Temperature  and  Latitude. — In  general,  the  mean  tem- 
perature of  the  air  decreases  as  latitude  increases.  In  the 
southern  hemisphere,  which  has  chiefly  an  ocean  surface,  the 
decrease  is  quite  regular  and  the  direction  of  the  isotherms 
does  not  vary  much  from  that  of  the  parallels.  In  the  northern 
hemisphere  the  decrease  is  by  no  means  regular,  and  the  iso- 
therms wanider  greatly  from  the  parallels. 

The  following  illustrates  the  decrease  in  the  United  States, 
as  affected  by  latitude.  In  column  I,  the  stations  from  south  to 
north  are  approximately  along  the  ninety-sixth  meridian;  in 
column  II  they  are  situated  along  the  Atlantic  Coast. 


I 

Corpus  Christi,  Tex 70°' 

Fort  Worth,  Tex 65° 

Wichita,  Kan 55° 

Lincoln,  Neb 50° 

Huron,  S.  Dak 42° 

Devils  Lake,  N.  Dak 36° 


II 

Key  West,  Fla 68° 

Savannah,  Ga 65° 

Wilmington,  N.  C 62° 

Atlantic  City,  N.  J 54° 

Boston,  Mass 49° 

Eastport,  Me 41° 


1  The  temperatures  noted  in  the  rest  of  this  chapter  are  Fahrenheit,  this 
scale  being  used  for  Weather  Bureau  reports, 


30 


THE   AIR:     THE    DISTRIBUTION   OF   WARMTH 


MEAN  TEMPERATURE  31 

I 

The  same  results  are  seen  in  the  temperature  range  in  Europe, 
from  Athens  to  Petrograd,  or  in  South  America,  from  Guayaquil 
to  Punta  Arenas. 

Within  the  tropics  and  also  in  polar  regions,  temperature 
changes  due  to  latitude  are  not  regular,  nor  are  they  great. 
In  the  main  they  are  due  to  causes  and  conditions  more  or  less 
local  in  character. 

Mean  Temperatures. — The  daily,  the  monthly  and  the 
yearly  means  are  required  in  weather  service.  The  most  ac- 
curate daily  means  would  require  the  average  of  the  hourly 
observations  for  the  day;  but  the  results  would  not  be  com- 
mensurate with  the  labor  involved.  The  investigations  of 
General  A.  W.  Greely,  while  at  the  head  of  the  U.  S.  Weather 
Bureau,  showed  that  the  average  deduced  from  readings  made 
at  7.00  A.M.,  2.00  P.M.,  and  9.00  P.M.,  taking  the  last  named  twice 
and  dividing  by  4  gave  a  result  very  closely  approaching  the  aver- 
age of  hourly  means.  This  method  has  much  to  recommend  it. 

In  the  various  Weather  Bureau  stations,  where  temperatures 
are  recorded  by  regular  observers,  and  in  the  various  military 
field  stations,  the  daily  mean  is  found  by  taking  half  the  sum 
of  the  daily  maximum  and  the  daily  minimum.  This  mean  is 
slightly  in  excess  of  the  mean  deduced  by  the  preceding  methods, 
but  the  error  is  so  small  that  it  may  be  disregarded. 

The  cooperative  observer's  day  begins  and  ends  with  the 
time  that  the  maximum  thermometer  is  set — usually  from  sun- 
set to  the  following  sunset.  The  daily  mean  thus  established, 
however,  does  not  differ  materially  from  the  true  mean.  The 
monthly  and  the  yearly  means  are  sufficiently  accurate  for 
practical  purposes. 

The  yearly  mean  is  deduced  by  dividing  the  sum  of  the 
monthly  averages  by  12.  A  closer  average  may  be  found  by 
adding  the  monthly  sums  and  dividing  by  the  number  of  days 
in  the  year  on  which  observations  are  made. 

The  mean  annual  temperature  of  a  region  is  not  a  key  to 
its  temperature  conditions  or  to  its  habitability.  Thus,  New 
York  City  and  San  Francisco,  both  seaports,  situated  not  far 
apart  in  latitude,  have  about  the  same  mean  yearly  tempera- 
ture. But  while  the  difference  between  the  winter  and  the 
summer  means  in  San  Francisco  is  not  more  than  8  degrees, 
in  New  York  it  is  about  32  degrees,  and  while  the  difference 


32 


THE   AIR:     THE    DISTRIBUTION   OF   WARMTH 


TEMPERATURE  RANGES  33 

between  the  warmest  and  the  coldest  month  in  San  Francisco 
is  10  degrees,  in  New  York  it  is  44  degrees. 

In  studying  the  temperature  of  a  locality,  therefore,  in 
addition  to  the  question  of  mean  annual  temperature,  various 
other  elements  must  be  taken  into  consideration.  In  the  main, 
these  are  the  daily  range,  the  range  of  monthly  means,  and  the 
seasonal  range.  These  are  affected  in  turn  by  latitude,  by 
altitude  above  sea  level,  by  the  direction  of  prevailing  winds, 
and  by  distance  from  the  sea.  In  a  minor  way  they  are  also 
affected  by  the  moisture  and  the  smoke  content  of  the  air. 

The  mean  annual  temperature  of  a  place  varies  but  little 
from  year  to  year.  In  New  York  City,  the  range  of  yearly 
means  has  varied  about  6  degrees  in  ninety-seven  years.  The 
average  of  each  ten-year  period  for  that  time  varies  but  a  trifle 
from  the  normal  of  52°.  The  records  of  Cooperstown,  New 
York,  have  been  kept  continuously  since  1854.  The  averages 
of  ten-year  periods  show  neither  apparent  gain  nor  loss  in 
temperature.1 

Temperature  Ranges. — The  daily,  monthly,  yearly  and  ex- 
treme ranges  all  have  an  important  bearing  on  the  climate  of 
a  region.  The  daily  range  is  a  part  of  the  records  of  every 
Weather  Bureau  station;  and  the  greatest  daily  range  in  each 
month  is  an  item  of  separate  record. 

The  various  ranges  are  usually,  though  not  at  all  stations, 
least  in  tropical  regions  and  greatest  in  inland  regions  where 
the  humidity  is  low.  In  temperate  latitudes  they  are  lower 
on  the  coasts  than  in  the  interior.  In  the  United  States  the 
average  daily  range  is  somewhat  less  along  the  Pacific  Coast 
than  along  the  Atlantic  Coast;  and  the  daily  ranges  of  inland 
stations  are  greater  than  those  of  coast  stations.  The  reason 
therefor  is  that  the  drier  air  of  inland  stations  permits  greater 
radiation  of  heat  at  night  and  greater  absorption  during  the 
day.  For  a  similar  reason  the  daily  ranges  at  stations  of  'con- 
siderable altitude  are  apt  to  be  greater  than  those  at  or  near 
sea  level. 

1  The  history  of  the  cultivation  of  the  grape  in  Europe  shows  even  n-ore 
conclusively  that  no  material  changes  have  occurred  in  two  thousand  years. 
The  grape  of  southern  and  western  Europe  is  semi-hardy;  it  likewise  is  sen- 
sitive to  temperature  changes.  But  in  twenty  centuries  its  limits  of  latitude 
have  not  changed. 


34  THE   AIR:    THE    DISTRIBUTION    OF    WARMTH 

Along  the  Atlantic  Coast  the  greatest  daily  range  within  a 
month  does  not  often  exceed  30  degrees;  and  the  average 
monthly  range  is  not  far  from  20  degrees.  Away  from  the 
coast  belt  daily  ranges  above  40  degrees  are  common.  In  the 
Plateau  Region  of  the  western  highlands  the  average  of  daily 
ranges  in  June  varies  from  40  degrees  to  50  degrees.  At  Pacific 
Coast  stations  the  average  daily  range  is  not  far  from  15  degrees. 
In  Arizona,  a  part  of  the  Plateau  Region,  owing  to  dry  air  and 
altitude,  the  daily  ranges  have  usually  been  greater  than  in 
most  other  parts  of  the  United  States.  At  Florence,  Arizona, 
a  daily  range  of  63  degrees  has  been  recorded,  and  ranges  above 
45  degrees  are  noted  occasionally. 

Undoubtedly  the  greatest  daily  ranges  occur  in  the  high 
desert  plateaus  of  Asia.  The  temperature  records  for  this 
region  are  few  in  number,  and  not  always  trustworthy.  One 
fact,  however,  has  been  established  beyond  reasonable  doubt: 
excessively  hot  days  are  sometimes  followed  by  freezing  tem- 
perature at  night. 

Excessive  extremes  are  characteristic  of  inland  regions; 
and  in  Siberia,  where  inland  distances  are  greater  than  in 
the  American  continent,  the  extremes  of  temperature  are  also 
greater.  At  Verkoyansk  a  minimum  of  —96°  F  has  been 
noted ;  and  at  Wargla,,  a  caravan  station  in  the  Sahara,  a  maxi- 
mum of  127°  F  has  been  reported  by  a  trained  observer.1  In 
the  northern  part  of  the  United  States,  a  temperature  of  —30° 
accompanying  a  cold  wave,  is  not  uncommon. 

In  the  United  States,  the  highest  official  temperature 
record,  134°,  is  reported  at  Greenland  Ranch,  California;  the 
lowest,  —67°,  at  Poplar  River,  Montana.  It  seems  certain  that 
desert  regions  in  low  latitudes  are  the  hottest  places  in  the 
world. 

Temperature  Normals. — The  daily  normals  of  a  station  are 
the  averages  of  each  day  of  the  year  for  a  period  of  not  less  than 
ten  years.  The  monthly  normal  is  the  average  for  the  par- 
ticular month  for  not  less  than  the  same  length  of  time;  the 
yearly  normal  is  computed  from  an  average  of  the  monthly 
normals.  It  is  the  custom  of  Weather  Bureau  stations  to 
extend  the  computation  of  the  means  to  the  end  of  succes- 

1  If  the  figures  are  authentic,  the  absolute  range  for  the  earth,  so  far  as 
is  known,  is  217  degrees. 


TEMPERATURE  AND  PREVAILING  WINDS  35 

sive  years 1  but  normals  once  established  seldom  change 
materially. 

It  is  the  custom  of  many  observers  to  note,  as  a  part  of  the 
daily  record,  the  number  of  degrees  above  or  below  the  daily 
normal;  this  is  the  "departure  from  the  normal."  If  below  the 
normal  the  number  is  prefixed  by  a  minus  sign.  It  is  an  excellent 
plan  to  carry  the  algebraic  sum  of  the  daily  departures  to  the 
end  of  the  year.  Many  of  the  daily  neswpapers  desire  these 
figures  as  a  matter  of  public  interest. 

The  monthly  normals,  by  comparison,  furnish  the  most 
instructive  data  concerning  the  temperature  conditions  of  a 
given  locality.  Thus  the  January  mean  at  Devils  Lake,  North 
Dakota,  is  o°;  at  New  Orleans  it  is  53°.  The  one  is  an  inland 
station  in  comparatively  high  latitude;  the  other  is  practically 
a  coast  station  in  much  lower  latitude.  The  January  mean  of 
San  Francisco  is  50°;  that  of  New  York  is  30°.  Both  are  coast 
stations,  but  San  Francisco  is  warmed  by  ocean  winds.  For  the 
eastern  part  of  the  United  States,  the  coast  stations  excepted, 
January  normals  are  not  far  from  30°,  and  July  normals  range 
from  70°  to  75°.  At  Moorhead,  Minnesota,  the  summer 
mean  is  67°;  at  San  Francisco,  58°;  at  Seattle,  63°;  at 
New  Orleans,  81°;  at  Key  West,  83°;  at  Yuma,  90°.  A  few 
stations  excepted,  January  is  the  coldest  and  July  the  warmest 
month. 

Temperature  and  Prevailing  Winds. —  Land  winds  are 
marked  by  great  ranges  in  temperature.  In  regions  far  from 
the  sea,  changing  winds  are  far  more  frequent  than  in  maritime 
regions.  Some  of  these  winds,  like  the  anticyclones  which  bring 
cold  waves,  are  widespread  in  prevalence;  others,  like  the 
simoon,  an  intensely  hot  and  dry  wind,  are  confined  mainly  to 
desert  regions. 

Throughout  the  greater  part  of  Europe  and  the  United 
States,  westerly  winds  prevail;  in  summer  they  are  frequently 
from  the  southwest,  and  in  winter  mainly  from  the  northwest. 
The  Pacific  Coast  of  the  United  States  receives  ocean  winds, 
and  the  winters  are  mild;  west  of  the  high  mountain  ranges 
zero  temperatures  rarely  if  ever  occur.  Along  the  coast,  summer 

1  Bulletin  R,  U.  S.  Weather  Bureau,  the  first  edition  in  1908,  contains 
the  normals  of  nearly  two  hundred  stations  computed  by  Professor  Frank  H, 
Bigelow.  The  changes  since  that  time  are  very  slight. 


36  THE    AIR:     THE    DISTRIBUTION   OF   WARMTH 

I 

temperatures  are  never  high;  towards  the  foot-hills  they  oc- 
casionally exceed  100°. 

East  of  the  Rocky  Mountains  moist,  southerly  winds  are 
common  during  the  summer  months,  and  occasionally  these 
extend  to  the  northern  border.  In  the  southern  half  of  the 
United  States  the  prevailing  winds  are  persistent,  moist,  and 
hot.  In  the  northern  part  they  are  not  so  moist,  but  very  warm. 
Rhode  Island  and  Delaware  possibly  excepted,  summer  tempera- 
tures of  1 00°  and  over  occur  in  every  state,  when  hot  westerly 
winds  prevail  for  a  few  days. 

It  is  obvious  that  sea  winds  are  more  equable  in  temperature 
than  land  winds.  Thus,  summer  days  in  San  Francisco  do  not 
often  reach  90°,  and  freezing  weather  occurs  perhaps  two  or 
three  times  in  a  decade.  When  such  temperatures  occur  they 
come  almost  always  with  land  winds.  The  normal  wind  at  this 
station  is  from  the  Pacific  Ocean.  In  New  York  City,  on  the 
other  hand,  prevailing  winds  are  land  winds;  and  within  a 
period  of  eight  months  a  range  of  115  degrees,  —13°  and  102°, 
has  occurred. 

Temperature  and  Radiation. — Very  dry,  clear  air  permits 
the  sun's  rays  to  pass  readily  to  the  earth  with  but  little  per- 
ceptible loss — that  is,  dry,  clean  air  is  diathermous  to  the  heat 
rays  that  impart  the  feeling  of  warmth  to  living  bodies.1  The 
heat  is  in  turn  absorbed  by  the  earth.  Earth  temperature 
at  the  surface,  or  to  a  depth  of  an  inch  or  two,  may  be  many 
degrees  higher  than  that  of  the  air.  Thus,  in  desert  regions, 
pieces  of  metal  lying  on  the  ground  in  the  sun  become  so  hot 
they  cannot  be  held  in  the  hand.  All  this  is  due  to  the  ab- 
sorption of  rays  to  which  the  air  is  diathermous. 

But  if  dry,  clean  air  permits  excessive  absorption,  it  also 

1  Not  all  the  heat  rays  impart  the  feeling  of  warmth;  in  many  instances, 
they  blister  and  burn  the  skin  without  imparting  this  sensation.  It  is 
thought  that  heat  of  this  character  consists  of  wave-lengths  which,  though 
they  may  destroy  living  tissue  of  certain  kinds,  do  not  stimulate  the  nerves  to 
which  the  temperature  sense  responds.  Thus,  in  popular  tradition,  there 
is  "sensible"  and  also  "insensible"  heat.  These  terms,  though  inexact, 
are  not  without  meaning.  On  dry,  winter  days,  the  flagstones  of  a 
sidewalk  frequently  absorb  enough  heat  to  melt  ice  and  snow,  even 
though  the  temperature  of  the  air  is  as  low  as  20°;  and  occasionally  side 
walks  and  hard-paved  streets  are  slushy  with  the  thermometer  scarcely 
above  25°. 


TEMPERATURE  AND  RADIATION  37 

permits  rapid  radiation.  The  nights,  in  regions  of  very  dry  air, 
may  be  bitterly  cold  although  mid-afternoon  has  been  in- 
tolerably hot;  indeed,  at  considerable  altitudes,  freezing  tem- 
peratures during  summer  nights  are  not  unknown  in  desert 
regions. 

Both  the  moisture  and  the  dust  and  smoke  content  of  the 
air  modify  the  absorption  of  the  sun's  heat  and  its  radiation 
by  the  earth.  The  moisture  and,  to  a  less  extent,  the  dust  and 
smoke  content  of  the  air  absorb  a  considerable  and  likewise 
a  measureable  part  of  the  heat  that  passes  readily  through 
dry  clean  air.  And  if  they  intercept  and  retard  the  passage 
of  heat  coming  to  the  earth,  they  also  retard  radiation  from  the 
earth  at  night.  In  other  words,  the  amount  of  insolation^ 
that  is,  of  solar  heat — received  at  the  earth's  surface  is  quite 
as  variable  as  is  the  daily  range;  indeed,  it  is  the  highest  ex- 
pression of  the  daily  range.  At  sea  level  dry  air  does  not  always 
indicate  warm  days  and  cool  nights;  but  at  levels  of  5000  feet 
or  more  this  is  the  rule  rather  than  the  exception.  At  any  level, 
changes  in  temperature  are  more  rapid  in  dry  than  in  moist 
air,  and  the  reason  therefor  is  obvious. 

Over  areas  of  moist  air  a  considerable  part  of  the  heat  of 
insolation  is  absorbed  in  another  way.  Almost  always  in  such 
areas  there  is  a  considerable  water  in  the  form  of  mist — that  is, 
minute  droplets  of  water.  When  the  sun's  warmth  converts 
these  to  vapor  the  absorbed  heat  becomes  latent  heat,  and 
no  longer  appears  as  sensible  heat.  This  fact  furnishes  another 
reason  why  the  air  over  desert  regions,  as  well  as  the  ground 
surface  itself,  becomes  heated  to  a  higher  degree.1 

Conditions  of  temperature  exercise  a  great  control,  not 
only  over  civilization,  but  over  the  distribution  of  life  itself. 
Humanity  may  overcome  its  environment  so  far  as  temperature 
is  concerned — man  can  command  fire,  food,  and  fuel  to  be 
brought  to  him ;  but  other  forms  of  life  cannot  rise  superior  to 
conditions  of  temperature.  The  line  beyond  which  grass  will 
not  grow  is  determined  in  part  by  temperature;  it  marks  the 
limit  beyond  which  grazing  animals  cannot  thrive,  and,  with  a 

1  A  moist  surface,  and  very  moist  air  as  well,  does  not  have  a  temper- 
ture  materially  higher  than  the  wet-bulb  thermometer;  a  dry  surface,  or 
very  dry  air,  acquires  a  temperature  approximating  that  registered  by  the 
black-bulb  thermometer. 


38  THE   AIR:    THE   DISTRIBUTION   OF   WARMTH 

few  exceptions,  cannot  survive.  Conditions  of  temperature, 
such  as  obtain  in  the  temperate  zones,  stimulate  both  the 
bodily  and  the  mental  faculties  of  humanity.1  Therefore  they 
have  resulted  in  a  civilization  fundamentally  different  from  that 
of  tropical  regions. 

1  "Every  species  of  plant  and  animal  has  an  optimum  temperature  at  which 
it  thrives  most  vigorously,  and  man  is  no  exception.  The  optimum  may  vary 
a  little  from  individual  to  individual,  but  not  much.  It  is  more  likely  to  vary 
from  one  type  of  activity  to  another.  For  physical  health  among  the  white 
race  as  a  whole,  the  best  temperature  is  an  average  of  64°  F  for  day  and  night 
together.  In  other  words,  people's  health  and  strength  are  greatest  when 
the  temperature  drops  to  about  56°  to  60°  at  night,  and  rises  to  somewhere 
between  68°  and  72°  during  the  middle  of  the  day.  For  mental  activity  the 
temperature  is  much  lower  than  for  physical,  being  an  average  of  approx- 
imately 40°.  In  other  words,  people's  minds  are  most  alert  and  inventive 
when  the  temperature  falls  to  about  freezing  at  night  and  rises  to  45°  or  50° 
by  day." — Huntington  and  Cushing's  Human  Geography. 


CHAPTER  V 
THE  AIR:    THE  DISTRIBUTION  OF  PRESSURE 

Measurement  of  Atmospheric  Pressure. — Practically  all  the 
movements  of  the  air  are  due  to  differences  in  its  temperature; 
but,  inasmuch  as  differences  in  temperature  result  in  differences 
in  the  density  of  the  air,  it  is  convenient  to  express  such  dif- 
ferences in  terms  denoting  the  force  with  which  the  air  presses 
upon  the  earth  at  sea  level.  It  is  also  more  convenient  in 
weather  science  to  express  this  pressure  in  terms  of  the  length 
of  a  column  of  mercury  which  the  air  balances — that  is,  a 
barometer.1 

Thus,  a  column  of  air  I  square  inch  in  cross-section  weighs 
at  sea  level  about  14.7  Ibs.,  or  I  atmosphere.  It  balances  a 
column  of  mercury  of  the  same  sectional  area,  29.92  inches  in 
length.  In  metric  terms,  the  weight  of  a  column  of  air  I  square 
centimeter  in  sectional  area  is  1033.3  grams,  and  it  balances  a 
column  of  mercury  760  millimeters  in  length.2  The  length  of  a 
column  of  water  which  balances  a  column  of  air  of  the  same 
sectional  area  is  about  34  feet. 

Distribution  of  Pressure. — The  movements  of  the  air  caused 
by  heating,  cooling,  expansion  and  contraction  include  the 
general  or  planetary  movements,  as  well  as  the  massing  of  the 
air  in  one  locality  and  the  counterbalancing  depressions  formed 
in  another.  The  expansion  of  the  air  by  heating  has  been  de- 
termined many  times.  If  1000  parts  of  air  at  32°  F  be  heated 
to  33°  F,  its  volume  will  be  increased  2.035  parts;  at  50°  F  the 
increase  will  be  36.63  parts;  at  130°  F,  a  temperature  very  com- 
mon under  a  summer  sun,  the  1000  parts  will  become  1199.43 

1  From  two  Greek  words  meaning  "measure  of  weight." 

2  British   observers  usually  express   the  pressure   of  the  atmosphere  in 
millibars:    I   inch  =33. 864    mb;     I   mb=  0.02953    inch;    therefore    at    29.53 
inches,  the  barometer  reading  is  1000  mb.     At  29.92  inches,  or  I  atmosphere, 
the  barometer  reading  is  1013.2  mb.     In  certain  computations,  the  millibar 
scale  of  the  barometer  possesses  many  conveniences. 

39 


THE  MRv   THE   DISTRIBUTION  OF   PRESSURE 


DISTRIBUTION  OF  PRESSURE  41 

parts.  In  centigrade  terms,  the  increment  is  0.00037  for  each 
degree,  measured  from  the  absolute  zero.  Small  as  seems 
the  rate  of  expansion,  the  actual  increment  over  a  continent, 
even  for  a  rise  of  temperature  of  a  few  degrees,  must  be  measured 
in  terms  of  cubic  miles,  and  its  aggregate  weight  in  millions  of 
tons. 

Observations  show  that  the  mean  pressure  over  the  earth 
varies  from  season  to  season;  at  any  given  locality  existing 
pressure  varies  from  hour  to  hour.  It  also  varies  according  to 
latitude;  but  the  variation  for  latitude  is  not  regular.  The 
maps  on  pages  40  and  42  show  the  several  regions  of  high 
pressure.  They  show  also  that  the  pressure  in  these  regions  in 
January  is  slightly  lower  than  in  July.  The  regions  of  summer 
high  pressure  are  situated  in  latitude  30°  to  35°  north  and 
south. 

The  southerly  regions  cover  the  ocean  and,  for  the  greater 
part,  are  far  removed  from  human  activities.  The  northerly 
regions  are  of  great  importance  from  the  fact  that  they  modify 
the  climate,  the  one  of  North  America,  the  other  of  Europe  and 
Asia. 

The  North  Pacific  high  covers  ocean  waters  in  July,  and  it 
tends  to  carry  cool  air  to  the  adjacent  coast.  In  January  it 
covers  the  western  part  of  Canada  and  at  times  pours  an 
enormous  volume  of  cold  air  over  the  greater  part  of  the  United 
States. 

The  North  Atlantic  high  covers  western  Asia  in  January; 
in  July  it  covers  the  ocean  east  of  the  United  States.  At  times, 
the  area  of  maximum  pressure,  30.30  inches  (1026  mb)  or  more, 
is  close  enough  to  the  continental  coast  to  retard  the  easterly 
flow  of  air  and  cause  a  pretty  general  stagnation  of  air  over  the 
eastern  part  of  the  United  States.  It  is  therefore  a  feature  in 
the  formation  of  hot  spells  over  that  region. 

The  area  covered  by  the  North  Pacific  has  a  January  pres- 
sure of  30.20  inches  and  a  July  pressure  of  30.30  inches  (1026 
mb).  The  Bermuda,  or  Atlantic  high,  is  about  30.25  inches 
(1024  mb);  over  Siberia,  however,  the  winter  high  is  not  far 
from  30.50  inches  (1033  mb).  It  is  thought  that  the  two  zones 
of  high  pressure  in  mid  latitudes  are  due  to  the  descent  of  the 
upper  currents  that  constituted  the  updraught  in  equatorial 
regions.  This  is  denied  by  many  meteorologists,  however. 


42 


THE  AIR:    THE  DISTRIBUTION  OF   PRESSURE 


MEAN  PRESSURE  OVER  THE  EARTH  43 

The  areas  of  low  pressure  are  much  larger  in  extent  than 
those  of  high  pressure  and,  as  a  rule,  they  are  not  so  well  de- 
fined. The  area  of  lowest  pressure  is  in  the  south  polar  region; 
it  is  inclosed  by  the  sixtieth  parallel;  its  mean  pressure  is  29.40 
inches  (996  mb) .  A  low  pressure  area  in  the  North  Atlantic 
lies  east  of  Greenland;  a  similar  low  pressure  area  in  the  North 
Pacific  covers  Bering  Sea. 

The  zone  of  ascending  air  currents  in  equatorial  regions  is 
a  region  of  low  pressure.  Its  mean,  summer  and  winter,  does 
not  vary  much  from  29.80  inches  (1009  mb).  Indeed,  the 
changes  in  pressure  from  month  to  month  throughout  tropical 
regions  are  very  slight.  The  ascending  currents  in  equatorial 
regions  and  the  descending  currents  near  the  tropics  are  as- 
sumed to  exist.  Their  existence,  established  circumstantially 
rather  than  positively,  explains  satisfactorily  the  position  of  the/ 
constant  highs  and  lows. 

The    great    differences    between    summer    temperature    and 
winter  temperature  explain  the  apparent  shifting  of  each  sum- 
mer high  from  a  position  over  the  ocean  in  summer  to  one  over 
the  nearby  continent  in  winter.     Cold  air  is  heavier  than  warm- 
air;   and,  in  the  latitude  of  the  constant  highs,  the  temperature 
of  the  air  over  the  land  in  winter  is  much  lower  than  over  the 
ocean;    in  the  summer,  on   the   other   hand,   the   temperature 
is  lower  over  the  ocean.     In  each   case  the  high  forms  in  the  - 
region  of  lower  temperature. 

Mean  Pressure  over  the  Earth. — It  is  customary  to  reduce 
all  pressure  observations  used  for  comparison  to  a  sea  level 
basis  and  to  a  temperature  of  32°  F  (o°  C).  The  maps,  pages 
40  and  42,  show  the  marked  variations  in  pressure  that  are 
seasonal.  From  these  pressures  the  mean  pressure  over  the 
earth  has  been  calculated  by  meteorologists  to  be  between 
29.90  and  29.85  inches.  W.  M.  Davis  has  calculated  the  mean 
pressure  over  the  northern  and  the  southern  hemispheres,  for 
the  summer  and  the  winter  months,  deducing  the  following 
values:  29.99  inches  (1016  mb)  for  January  and  29.87  inches 
(1012  mb)  for  July  in  the  northern  hemisphere;  2991  inches 
(1013  mb)  in  July  (midwinter)  and  29.79  inches  (1009  mb)  in 
January  (midsummer)  in  the  southern  hemisphere. 

The  determination  of  mean  pressure  over  each  of  the  two 
hemispheres  is  more  important  than  that  of  the  mean  pressure 


44  THE  AIR:    THE  DISTRIBUTION  OF  PRESSURE 

of  the  earth  as  a  whole.  The  fact  that  the  pressure  over  the 
southern  hemisphere  is  lowest  at  the  time  when  it  is  highest 
over  the  northern  hemisphere,  and  vice  versa,  indicates  the 
shifting  of  an  enormous  volume  and  weight  of  air  from  one 
hemisphere  to  the  other,  twice  a  year.  Davis  estimates  that 
the  weight  of  air  thus  moved  is  equivalent  to  a  pressure  of  0.12 
inch  (4.1  mb),  or  between  30  and  35  million  tons  in  weight. 

Density. — It  is  evident  that  a  close  relation  exists  between 
the  temperature,  pressure  and  density  of  the  air.  With  lowering 
temperature,  the  volume  of  air  contracts,  and  air  flows  in  to 
equalize  the  loss.  The  cold  air  contains  a  greater  number  of 
molecules  per  given  volume  and  therefore  its  density  is  in- 
creased; because  it  contains  more  matter  per  given  volume  its 
weight,  and  therefore  its  pressure  is  increased.  Density  varies 
directly  with  pressure  and  inversely  with  temperature.  It  also 
varies  inversely  with  the  moisture  content  of  the  air. 

For  the  greater  part,  human  activities  are  carried  on  at  the 
plane  of  contact,  where  earth  and  air  meet.  Weather  science, 
however,  includes  a  study  of  density  and  pressure  at  all  ob- 
servable altitudes  from  sea  level  upward.  Density  of  the  air 
at  different  heights  also  affects  air  flight  and  the  flight  of  pro- 
jectiles. Hence,  a  knowledge  of  the  density  of  the  air  at  dif- 
ferent altitudes  is  necessary. 

The  changes  in  the  density  of  the  air  are  most  marked  at  the 
earth's  surface.  The  daily  range  in  density  may  be  as  much  as 
10  per  cent,  and  the  extreme  range  in  a  year  has  exceeded  20 
per  cent.  The  range  is  greatest  in  temperate  latitudes  at  or 
near  sea  level.  The  changes  in  density  due  to  temperature 
variations  explain  the  high  midwinter  pressure  over  inland 
regions  and  the  high  midsummer  pressure  in  oceanic  regions; 
the  rock  envelope  of  the  earth  radiates  its  heat  more  rapidly 
than  does  the  water.  Very  low  temperatures  in  winter  increase 
the  density  of  the  air.  High  temperatures  in  summer  decrease, 
the  density,  with  the  result  that  oceanic  regions  are  cooler 
in  summer  and  warmer  in  winter  than  far-inland  continental 
regions. 

Diurnal  and  Semi-Diurnal  Changes  in  Pressure. — At  sta- 
tions of  considerable  elevation  a  maximum  daily  pressure  at 
the  warmest  part  of  the  day  is  observable.  It  is  attributed  to 
the  heating  of  the  air,  thereby  causing  an  accumulation  which 


DIURNAL  AND  SEMI-DIURNAL  CHANGES  IN  PRESSURE     45 


practically  forms 
the  crest  of  the 
wave  of  greatest 
warmth.  During 
the  coldest  hours  of 
the  day  a  reverse 
movement  takes 
place  and  forms  a 
corresponding 
trough  of  pressure. 
This  diurnal  maxi- 
mum ^  an<3  mini- 
mum of  pressure  is 
practically  a  raising 
and  lowering  of  the 
center  of  mass.  As 
a  result,  a  greater 
mass  means  greater 
pressure,  and  vice 
versa. 

The  semi-diur- 
nal maximum  and 
minimum  is  very 
regular  and  obtains 
in  every  part  of  the 
earth.  It  is  best 
studied  from  the 
barogram,  a  strip 
of  paper  attached 
to  the  revolving 
drum  of  a  record- 
i  n  g  barometer. 
The  line  drawn  by 
the  barograph  pen 
shows  a  slight  rise 
above  mean  pres- 
sure at  10  o'clock, 
morning  and 
night,  followed  by 
a  depression  a  t 


46  THE  AIR:    THE   DISTRIBUTION  OF   PRESSURE 

4  o'clock,  morning  and  afternoon.  These  oscillations  in  pressure 
are  probably  due  to  the  waves  of  temperature  which  ceaselessly 
follow  the  sun  with  the  rotation  of  the  earth. 

The  semi-diurnal  maxima  and  minima  are  greatest  in 
equatorial  latitudes;  they  decrease  in  higher  latitudes.  Ac- 
cording to  the  observations  of  General  Greely,  the  oscillations 
of  the  barograph  pen  were  scarcely  noticeable  in  polar  regions. 
The  amplitude  of  oscillation  is  greater  by  day  than  by  night; 
it  is  greater  at  the  equinoxes  than  at  the  solstices.  The  day 
amplitude  is  greater  over  the  continents  than  over  the  sea; 
the  night  amplitude  is  the  reverse.  According  to  Humphreys, 
the  whole  atmospheric  shell  vibrates  in  waves  which  happen 
to  be  in  1 2-hour  wave  lengths.  Records  made  by  P.  R.  Jameson 
on  the  East  African  Coast  near  the  equator  show  a  maximum 
of  about  0.025  mcn  above  and  the  same  minimum  below  normal 
pressure.1  In  tropical  regions  the  irregular  variations  in  pres- 
sure are  infrequent;  the  semi-diurnal  oscillations,  on  the  other 
hand,  are  very  regular.  The  claim  that  an  observer  can  tell 
the  time  of  day  by  the  barometric  pressure  is  not  without 
foundation. 

Other  Variations  in  Pressure. — Pressure  ranges  exceeding 
1.5  inches  during  a  week — the  record  of  a  barograph  sheet — 
are  not  uncommon,  .  Weekly  and  monthly  ranges  are  usually 
much  greater  in  winter  than  in  summer  and  much  greater  in 
mid-latitudes  than  in  low  or  high  latitudes.  Professor  Mohn 
has  summarized  as  follows:2  The  barometer  is  high  when  the 
air  is  cold,  when  it  is  dry,  and  when  an  upper  current  flows 
into  a  given  area.  It  is  low  when  the  lower  air  is  heated,  when 
it  is  damp,  and  when  it  has  an  upward  movement. 

The  variations  in  pressure  with  which  weather  science  is 
chiefly  concerned  are  the  daily  highs  and  lows  which  cross  the 
continents  in  mid-latitudes  from  west  to  east  and,  for  the 
greater  part,  are  lost  in  mid-ocean.  These  great  billows  of  the 
atmosphere  are  comparable  to  the  billows  of  the  sea;  but,  as 
is  shown  in  Chapter  XIII,  the  lows  are  usually  storm  centers  and 
the  highs  are  rapidly  moving  masses  of  cold  air.  The  former 
are  the  cyclones  of  the  forecaster;  the  latter,  the  anticyclones. 

1  At  Mount  Vernon,  N.  Y.,   Lat  40°  54',   the  values  are  approximately 
0.022  inch — possibly  less. 

2  Grundziige  der  Meteorologie. 


ISOBARS  AND  GRADIENTS  47 

When  accompanied  by  rain  or  by  snow  they  are  the  "storms"  of 
popular  tradition.  Local  disturbances,  such  as  tornadoes, 
water  spouts  and  thunder-storms,  usually  affect  pressure,  and 
leave  each  its  record  on  the  sheet  of  the  recording  barometer. 
Observers  learn  quickly  to  interpret  these  records. 

Isobars  and  Gradients. — The  distribution  of  pressure  is  best 
shown  by  means  of  lines  drawn  on  a  map  through  adjacent 
points  having  the  same  pressure.  These  lines  are  isobars.  The 
maps,  pp.  40  and  42,  show  midwinter  and  midsummer  isobars. 
For  the  sake  of  comparison,  the  figures  are  reduced  to  the  basis 
of  sea  level  and  temperature  of  32°  F  (o°  C).  The  isobars  on 
the  daily  weather  map  show  the  respective  positions  of  highs 
and  lows,  and  from  them  the  daily  forecasts  are  made. 

On  the  daily  weather  map  the  conditions  of  pressure  are 
interpreted  by  a  study  of  the  relative  positions  of  the  isobars, 
which  constitute  a  contour  map  of  the  air.  If  the  isobars  of  a 
high,  or  of  a  low,  are  close  to  one  another,  the  slope  of  the  air 
wave  is  steep;  if  far  apart,  the  slope  is  gentle.  Therefore,  in 
either  case,  they  show  the  gradient  of  pressure,  and  from  the 
gradient  of  pressure  an  approximate  velocity  of  the  wind  may 
be  indicated. 

About  half  a  century  ago,  Whipple,  of  Kew  Observatory, 
prepared  an  empiric  table  based  upon  isobars  15  nautical 
miles  apart — that  is,  a  1 5-mile  gradient.  If,  for  instance,  the 
gradient  is  o.i  inch,  the  indicated  velocity  of  the  wind  will  be 
approximately  9  miles  per  hour;  if  0.2  inch,  it  will  be  about 
17  miles  per  hour,  etc.  The  study  of  the  pressure  gradient,  there- 
fore, is  a  fairly  accurate  indication  of  wind  velocity.  It  also 
enables  the  observer  to  make  a  reasonably  accurate  forecast  of 
wind  velocity  from  twenty-four  to  thirty-six  hours  in  advance. 

Actual  and  Recorded  Pressure. — Pressure  decreases  with 
altitude,  at  a  varying  rate.  If  the  lowest  of  a  pile  of  ten  books 
is  lifted,  the  weight  of  the  nine  books  above  it  must  be  over- 
come; but  if  the  fifth  book  is  lifted,  the  weight  of  only  four 
books  must  be  overcome.  The  same  principle  applies  to  the 
atmosphere.  At  sea  level  a  column  of  air  I  square  inch  in 
cross-section,  presses  with  a  weight  of  14.7  Ibs.,  but  at  a  height 
of  19,000  feet  the  weight  of  the  column  is  only  half  as  great. 
For  the  first  few  hundred  feet  above  sea  level  the  pressure 
decreases  at  the  rate  of  o.i  inch  for  each  90  feet  of  ascent;  at 


48  THE  AIR:    THE  DISTRIBUTION  OF   PRESSURE 

an  altitude  of  3000  feet  it  is  at  the  rate  of  o.i  inch  for  100  feet 
of  ascent.  The  greater  number  of  weather  stations  in  the 
United  States  are  1000  feet  or  more  above  sea  level,  and  many 
of  those  west  of  the  Denver  meridian  are  more  than  5000  feet 
above  sea  level. 

For  purposes  of  comparison  in  the  preparation  of  daily 
weather  maps,  all  pressure  observations  are  reduced  to  sea 
level  basis.  For  this  purpose  such  a  reduction  is  necessary, 
and  all  reduced  pressures  within  an  altitude  of  a  few  hundred 
feet  of  sea  level  are  sufficiently  correct  for  practical  purposes. 
For  altitudes  materially  greater  than  1000  feet,  the  results 
when  applied  to  mean  pressure,  are  erroneous.  Thus,  at  Mount 
Washington,  the  mean  recorded  pressure  for  January,  reduced 
to  sea  level,  is  greater  than  that  for  July.  As  a  matter  of  fact, 
the  actual  mean  pressure  is  less  in  January  than  in  July.  The 
following  illustration  will  explain: 

A  compress  12  feet  in  height  is  filled  with  loose  cotton.  The 
pressure  of  its  weight  at  the  bottom  is,  say,  16  Ibs.  per  square 
foot.  Half  way  to  the  top,  at  the  6-foot  level,  the  pressure  is  half 
as  much.  Now  let  us  assume  that  the  cotton  is  compressed  so 
that  its  depth  is  only  9  feet.  The  pressure  at  the  bottom  re- 
mains the  same;  but  at  the  6-foot  level,  there  is  only  half  as 
much  cotton  as  before  compression;  hence  the  pressure  is 
half  as  great.  The  center  of  mass  has  been  lowered  in  the 
process  of  compression. 

The  same  principle  applies  in  the  case  of  measurements  of 
the  atmosphere.  Thus,  at  Mount  Washington,  and  at  other 
stations  of  considerable  altitude,  expansion  due  to  tempera- 
ture-increase raises  the  center  of  mass  in  summer;  mean  pres- 
sure, therefore,  is  raised.  In  winter,  low  temperature  causes 
contraction,  lowering  the  center  of  mass  and  therefore  the 
pressure.  In  other  words,  while  sea  level  and  also  the  ob- 
server's station  are  at  fixed  altitudes,  the  center  of  mass  is  a 
varying  altitude;  it  is  raised  by  increasing  temperature  and 
lowered  by  decreasing  temperature.  Hence  its  effect  on  mean 
pressure. 


CHAPTER  VI 
THE  AIR:    MAJOR  CIRCULATION:    LOCAL  WINDS 

The  convectional  or  vertical  movements  of  the  air  have 
been  mentioned  incidentally  as  affecting  the  diffusion  of  warmth. 
They  are  considered  in  detail  in  succeeding  chapters.  The  hori- 
zontal movements  constitute  the  winds.  So  far  as  human 


Red-way's  Physical  Geography. 
General  movements  of  the  atmosphere. 

activities  are  concerned,  the  general  horizontal  movements 
consist  of  a  broad  tropical  belt  of  easterly  winds  flanked  on  the 
north  and  on  the  south  by  a  broad  belt  of  westerly  winds.  The 
three  belts  move  northward  with  the  apparent  motion  of  the 
sun  northward  in  June,  and  southward  in  December.  The 
yearly  oscillation  covers  about  120  degrees  of  latitude,  a  total  of 
about  7200  nautical  miles. 


50          THE    AIR:    MAJOR  CIRCULATION;    LOCAL  WINDS 

Trade  Winds. — The  broad  belt  of  easterly  winds  within  the 
tropics  is  popularly  known  as  the  Trade  Winds.  Their  direction  is 
southwesterly  in  the  northern  and  northwesterly  in  the  southern 
part  of  the  belt.  The  heating  of  the  air  in  equatorial  regions 
causes  a  convectional  updraught;  and  this  is  balanced  by  an 
inflow  of  air  from  higher  latitudes.  The  rotation  of  the  earth 
deflects  the  movement  of  the  air,  giving  a  westerly  motion  to 
the  winds.  Their  force  and  direction  are  best  studied  from  the 
monthly  pilot  charts  published  by  the  United  States  Hydro- 
graphic  Office.  Although  the  pilot  charts  refer  specifically  to 
ocean  winds,  the  general  information  published,  so  far  as  wind 
direction  is  concerned,  applies  to  land  winds  also. 

The  strength  of  the  Trade  Winds  varies  according  to  latitude 
and  also  according  to  season.  The  velocity  is  highest  near  the 
edges  of  the  belt  and  lowest  at  its  center;  it  varies  from  about 
8  miles  per  hour  in  the  fall  months  to  about  twice  this  rate  in 
the  spring  and  summer  months.  The  southeast  winds  are 
materially  stronger  than  the  northeast  winds.  The  easterly 
component  is  the  important  commercial  factor — hence  the 
popular  name.  For  the  year  their  average  is  from  n  to  14 
miles  per  hour,  or  from  2  to  3,  Beaufort  scale.1  On  the  Pacific 
Ocean  the  Trade  Winds  are  neither  so  strong  nor  so  regular 
as  in  the  Atlantic;  on  the  Indian  Ocean  only  the  southern  part 
of  the  belt  is  observable. 

Prevailing  Westerlies. — The  two  broad  belts  which  flank 
the  Trade  Winds  are  variously  named  "Counter  Trades," 
"Return  Trades,"  and  "Anti-Trades."  Their  direction  varies— 
northeast,  east,  and  southeast  as  shown  on  the  pilot  charts, 
and  their  strength  is  indicated  by  the  arrows.  In  the  southern 
hemisphere  because  of  their  strength,  they  are  known,  as  the 
"Roaring  Forties."  In  the  days  of  sailing  vessels,  a  ship  from  a 
port  of  Europe  to  Australia  could  usually  make  the  return  trip 
more  expeditiously  by  way  of  Cape  Horn.  The  force  of  the 
Prevailing  Westerlies  is  from  2  to  4,  Beaufort  scale. 

The  Prevailing  Westerlies  begin  as  an  upper  wind  in  Trade 
Wind  latitudes,  descending  to  sea  level  at  the  edges  of  the 
Trade  Wind  belt,  approximately  Lat.  30°  N.  and  S.  Over  Cuba 
the  airman  may  find  them  at  the  height  of  about  11,500  feet; 
over  Jamaica  about  19,500  feet;  and  over  Trinidad  about 

1  Table,  page  240. 


WINDS  OF  THE  UNITED  STATES 


51 


26,000  feet.    In  each  case  the  height  of  the  Prevailing  Westerlies 
is  also  the  depth  of  the  Trade  Wind  belt. 

Winds  of  the  United  States. — The  main  body  of  the  United 
States  is  situated  in  the  belt  of  Prevailing  Westerlies.  The 
prevailing  surface  winds  therefore  are  northwest,  west,  and 
southwest.  East  of  the  Missouri  River  northwest  winds  prevail 
except  during  the  hottest  part  of  summer  when  southwest  winds 
are  the  rule.  These  include  practically  all  the  winds  of  various 
altitudes  between  sea  level  and  the  summit  of  Mount  Wash- 


Tracks  of  cyclonic  storms  preceded   by  easterly  and   followed  by  westerly 

winds. 

ington  (6293  feet),  one  of  the  highest  points  east  of  the 
Mississippi  River. 

South  of  the  thirty-first  parallel  the  influence  of  the  Trade 
Winds  is  very  apparent,  and  the  prevailing  winds  in  summer 
vary  from  northeasterly  to  easterly.  Along  the  coast  this 
influence  extends  much  higher  than  the  thirty-first  parallel, 
and  northeasterly  fair  weather  winds  occur  at  times  as  far  north 
as  the  Maine  Coast. 

West  of  the  Mississippi  River  to  the  Rocky  Mountains,  the 
winds  vary  from  southwesterly  to  northwesterly.  They  are 
steadier  and  stronger  than  those  east  of  the  Mississippi  River. 
Southwesterly  winds  prevail  much  of  the  time. 


52         THE  AIR:    MAJOR   CIRCULATION;    LOCAL  WINDS 

From  the  Atlantic  Coast  to  the  Rocky  Mountains  the 
general  westerly  direction  of  the  winds  prevails  pretty  steadily, 
except  as  it  is  upset  occasionally  by  cyclonic  storm  winds. 

In  the  plateau  region  and  the  basin,  the  upper  winds  are 
westerly — southwest  to  northwest;  but  the  surface  winds  in 
many  instances  are  deflected  by  mountain  ranges  and  become 
either  southerly  or  northerly. 

The  surface  winds  of  the  Pacific  Coast  are  westerly;  but  in 
the  Sacramento  and  San  Joaquin  valleys  they  become  northerly 
or  southerly,  being  deflected  by  the  mountain  ranges.  When 
the  temperature  of  the  interior  is  high,  strong  westerly  winds 
are  the  rule  along  the  coast.  This  is  especially  noticeable  in 
the  vicinity  of  San  Francisco. 

Monsoons. — The  monsoons  are  seasonal  winds  which 
blow  from  the  sea  over  the  land  during  summer  months  and  in 
an  opposite  direction  during  winter  months.  The  name,  mean- 
ing "season,"  was  first  applied  to  the  seasonal  winds  of  the 
Indian  Ocean  Coast;  subsequently  it  was  applied  to  various 
seasonal  winds  of  ocean  coasts.  In  southern  Asia  the  crop 
yield  depends  very  largely  on  the  rainfall  which  accompanies 
the  southwest  monsoon;  hence  its  importance  in  the  economic 
history  of  a  considerable  part  of  southern  Asia. 

The  advance  and  recession  of  the  Trade  Wind  belt  along  the 
Gulf  Coast  of  the  United  States  seems  to  emphasize  a  similar 
alternation  of  sea  wind  and  land  wind ;  but  the  monsoon  charac- 
teristics extend  as  far  north  as  Long  Island  Sound  on  the  north 
and  far  into  Mexico  on  the  south.  In  the  latitude  of  New  York 
City,  about  eight  weeks  of  southwesterly  winds  prevail  in 
summer,  while  northwest  winds  prevail  the  rest  of  the  year. 

Calm  Belts. — Along  the  narrow  belt  where  the  northeast 
and  the  southeast  Trade  Winds  meet,  the  easterly  components 
of  the  winds  disappear,  the  only  movement  being  an  updraught. 
In  the  days  of  sailing  vessels  ships  sometimes  lay  becalmed  for 
many  days — hence  the  expressive  name,  Doldrums.  This  calm 
belt  lies  north  of  the  equator  and  practically  covers  the  thermal 
equator.  It  is  a  region  of  low  barometer,  very  moist  air, 
cumulus  clouds  and  excessive  rains.  It  is  practically  coincident 
with  the  tropical  rain-belt.  Its  detrimental  effects  on  marine 
transportation  ceased  with  the  advent  of  steam  navigation. 
Years  ago  it  was  a  terror  to  sailing  craft. 


LOCAL  WINDS 


53 


The  Calms  of  Cancer  and  of  Capricorn  separate  the  belts  of 
prevailing  westerly  winds  from  the  Trade  Wind  belt.  They  are 
regions  of  high  pressure  and  usually  of  cloudless  skies.  Vessels 
from  ports  of  Europe  and  the  United  States  crossed  the  Calms 
of  Cancer  when  making  West  Indian  ports.  These  calms  were 
therefore  a  great  drawback  to  commerce.  Steam  navigation 
has  eliminated  the  waste  of  time  and  the  loss  of  jettisoned 
cargoes;1  but  conditions  more  or  less  detrimental,  which  hu- 
manity cannot  overcome,  still  exist.  Much  of  the  southwestern 


January 


July 

Red-way's  Physical  Geography. 


Winds  of  the  Atlantic. 


part  of  the  United  States  and  northern  Mexico  are  covered  by 
the  high-pressure  Calms  of  Cancer,  and,  at  a  little  distance 
from  the  ocean  coasts,  rain-bearing  winds  are  infrequent. 
Similarly,  the  sparse  rainfall  of  parts  of  South  America  is  due 
to  the  Calms  of  Capricorn. 

Local  Winds. — The  local  winds  of  a  region  appeal  to  a  com- 
munity more  forcibly  than  do  the  general  movements.  One 
may  not  appreciate  the  fact  that  the  habitability  of  a  region 

1  On  various  occasions  vessels  whose  cargoes  consisted  of  horses  were 
becalmed  in  this  region.  When  the  supply  of  water  gave  out  the  horses 
were  thrown  overboard — hence  the  name  "horse  latitudes." 


54         THE  AIR:    MAJOR   CIRCULATION;    LOCAL  WINDS 

depends  very  largely  upon  the  general  movements  of  the  air; 
but  no  one  can  fail  to  realize  the  importance  of  a  hot  blast,  a 
blizzard,  a  tornado  or  a  sand  storm — or,  indeed,  of  any  oc- 
casional storm  wind  that  may  injure  growing  crops  and  destroy 
property. 

Along  coasts,  the  sea  breeze  and  the  alternating  land  breeze 
are  the  rule  rather  than  the  exception  during  a  consider- 
able part  of  the  year.  As  a  rule,  the  sea  breeze  extends  rarely 
higher  than  3000  feet.  At  such  times  it  may  be  merely  a  cross- 
wind,  and  the  clouds  at  a  height  of  a  mile  may  be  moving  in 
an  opposite  direction.  The  succeeding  land  breeze  which  sets 
in  is  apt  to  be  a  much  stronger  wind. 

Mountain  Valley  Winds  are  common  in  all  mountain  regions. 
During  the  day,  when  the  air  is  growing  warmer,  the  wind 
blows  up  the  valley;  at  night,  when  it  is  losing  its  heat  the 
flow  is  down  the  valley.  In  narrow  canyons,  the  night  winds 
may  be  very  strongj — a  force  of  6  to  7  of  the  Beaufort  scale. 

The  Chinook,  one  of  the  most  important  local  winds,  derives 
its  name  from  the  jargon  of  a  tribe  of  Indians  living  near  the 
mouth  of  the  Columbia  River.  According  to  tradition  the  name 
means  "snow-eater,"  from  the  fact  that,  with  its  appearance,  the 
snow  begins  to  melt  first  from  the  higher  parts  of  the  mountain 
slopes  and,  last  of  allv  from  flood  plains  and  valley  floors. 

The  Chinook  was  made  known  first  by  early  settlers  in 
Oregon.  In  time  it  was  found  to  exist  throughout  much  of  the 
montane  part  of  the  northwest.  The  Chinook  begins  as  a  moist 
wind  on  the  windward  side  of  a  high  range.  As  it  is  pushed 
upward  along  the  mountain  slope  it  is  chilled  by  expansion  below 
the  dew-point,  and  condensation  takes  place.  This  liberates  a 
great  deal  of  latent  heat,  materially  warming  the  air.  The  air 
is  warmed  still  further  by  compression  as  it  rolls  down  the  lee- 
ward slope  of  the  range. 

In  Montana,  Idaho  and  Alberta,  the  Chinook  wind  is  far- 
reaching  in  its  climatic  effects.  Both  grazing  and  wheat-grow- 
ing are  made  possible  in  regions  that  otherwise  would  be  un- 
productive. The  Chinook  wind  does  not  differ  from  the  Foehn 
wind  of  Europe  with  which  it  is  classed.  In  each  case  moist 
air  drawn  into  a  cyclone  and  pushed  over  a  range,  descends 
on  the  other  side  as  a  warm,  dry  wind. 

The   Hot  Winds  of  the  Plains,  including  the  Summer  Winds 


LOCAL  WINDS  55 

of  Texas,  and  the  Norther  of  the  San  Joaquin-Sacramento 
Valleys  are  classed  among  the  "destroyers,"  from  the  fact  that, 
in  many  localities,  two  or  three  days  of  their  duration  is  fatal 
to  growing  crops. 

The  Santa  Ana  of  southern  California  is  the  outpouring  of 
a  hot,  dust-laden  desert  wind  through  one  or  more  of  the 
mountain  passes.  In  the  past  thirty-five  years,  irrigation  and 
cultivation  have  been  extended  into  the  arid  region,  with  a  result 
that  the  Santa  Ana  is  largely  deprived  of  its  dust  content  and 
its'  high  temperature.  The  Santa  Ana  in  its  old  time  vigor  was 
merely  the  edge  of  a  desert  simoon  that  intruded  upon  nearby 
fertile  lands.  The  simoon  itself  occurs  in  every  desert  so  far 
as  is  known.  It  is  a  sand  storm  because  of  its  velocity.  In  the 
Colorado  and  Mohave  deserts  the  simoon  may  have  a  velocity 
exceeding  75  miles  an  hour.  The  Washoe  Zephyr  of  the  Basin 
Region  of  the  United  States,  and  the  Khamsin  of  Egypt  are 
desert  winds  of  the  same  kind.  They  are  thought  to  be  cy- 
clonic in  character,  but  practically  they  are  dust-laden  winds, 
either  blowing  into  a  desert,  or  out  of  a  desert. 

The  Texas  Northers  are  biting  cold  winds,  common  to  the 
high  western  plains  of  the  United  States  and  northern  Mexico. 
They  usually  follow  warm  and  balmy  winds  of  southerly  direc- 
tion. The  onset  may  be  very  sudden.  A  fall  of  temperature 
of  50  degrees  within  a  day  is  not  uncommon.  The  Bora  and 
the  Mistral  of  the  Mediterranean  coast  of  Europe  are  similar 
in  character;  they  are  cold  winds  sliding  down  the  steep 
mountain  slopes  because  of  increasing  pressure  to  the  north- 
ward. In  the  southern  hemisphere,  the  Pampero  is  the  counter- 
part. It  is  most  noticeable  in  the  pampas,  or  great  plains  east  of 
the  Andes,  and  in  many  instances  it  extends  to  the  coast.  Al- 
though a  southwest  wind,  it  is  classed  with  Northers  because 
of  its  origin.1  The  Blizzard  is  nominally  a  cold-wave  wind  which 
is  sufficiently  vigorous  to  pick  up  and  carry  loose  snow;  it  is  a 
northwesterly  wind.  Popular  usage  applies  the  name  to  any 
wind  of  gale  force  that  accompanies  a  snowstorm. 

Direction  and  Velocity. — The  diagram  of  the  major  circula- 
tion of  air  shows  that  the  normal  movements  of  winds  are  north- 

1  The  name  is  also  applied  to  the  "squall"  type  of  descending  wind 
accompanied  by  thunder  and  lightning,  occasioned  in  the  pampas  of  South 
America. 


56         THE  AIR:     MAJOR   CIRCULATION;    LOCAL   WINDS 

west,  southwest,  northeast,  and  southeast  and  that  winds  from 
the  north,  south,  east  or  west  are  the  exception.  Winds  over 
the  land,  however,  are  apt  to  be  modified  by  local  topography; 
and  it  frequently  happens  that  a  surface  wind  differs  materially 
in  direction  from  the  wind  at  low  cloud  heights. 

Throughout  the  eastern  half  of  the  United  States,  about 
half  the  recorded  mileage  of  the  wind  is  from  points  between 
north  and  west.  Along  the  Gulf  Coast  to  a  distance  of  about 
300  miles  inland,  winds  with  a  southerly  element  of  movement 
prevail.  As  a  rule,  the  winds  are  strongest  during  the  winter 
months  and  mildest  in  summer.1  For  the  greater  part,  the 
prevailing  winds  of  the  Pacific  Coast  region  are  northwesterly. 
At  San  Diego  about  two-thirds  of  the  mileage  is  recorded  by 
winds  blowing  between  west  and  north. 

The  strongest  winds  are  apt  to  occur  along  the  coasts  of 
the  sea  and  the  Great  Lakes.  The  mean  hourly  velocity  at 
Sandy  Hook,  Block  Island,  Delaware  Breakwater  and  Cape 
Mendocino  exceeds  14  miles.  Throughout  the  plains  west  of 
the  Missouri  River,  high  winds  prevail.  The  long  downward 
slope  over  a  smooth  surface  adds  to  the  velocity  of  westerly 
winds.  During  winter  months  the  cold-wave  winds  from 
Canada  contribute  a  mass  of  air  which,  moving  eastward,  gives 
added  velocity  to  the  winds  of  this  region. 

The  latitude  of  strongest  winds  in  the  United  States  is 
approximately  along  the  forty-fifth  parallel  in  the  summer  and 
a  few  degrees  lower  in  winter.  Winter  months  are  the  season 
of  the  strongest  winds.  The  winds  of  greatest  strength,  however, 
are  storm  winds — winter  cold-wave  blasts,  or  the  recurved  por- 
tion of  West  Indian  hurricanes  which  sweep  northward  along 
the  Atlantic  Coast. 

Other  Features  of  General  Circulation. — The  foregoing 
paragraphs  present  a  very  elementary  view  of  the  greater 
circulation  of  the  air.  As  a  matter  of  fact,  not  much  is  known, 
even  of  the  surface  winds  over  a  very  large  part  of  the  earth. 

1  The  records  of  about  twenty  stations  in  the  northeast  quarter  of  the 
United  States  show  the  following  mean  velocities  in  miles  per  hour  for  the 
year: 


Northwest 8.8 

West 4.6 

Southwest 5.2 


Northeast 5.3 

East 4.7 

Southeast 4.8 


OTHER  FEATURES  OF  GENERAL  CIRCULATION  57 

Until  within  the  past  few  years,  knowledge  of  the  upper  winds 
was  imperfect  and  fragmentary.  Sounding  balloons  and  kites 
furnished  with  recording  apparatus  are  beginning  to  supply 
humanity  with  much-needed  information  concerning  horizontal 
movements  of  the  upper  air;  the  airmen  are  furnishing  knowl- 
edge of  vertical  movements. 

Research  in  recent  years  shows  that  the  updraught  in 
equatorial  regions  is  not  uniform  in  force  nor  continuous  at  all 
times.  Neither  is  the  overflow  of  rising  air  toward  polar  regions 
uniform  or  regular.  Sounding  balloons  occasionally  have  been 
carried  toward  the  equator  instead  of  away  from  it.  Stiff  west 
winds  also  have  been  observed  in  equatorial  regions  at  the  height 
of  a  few  thousand  feet,  surmounted  by  easterly  winds  at  a  still 
greater  elevation. 

Sounding  balloons  do  not  find  the  decrease  of  temperature 
with  increasing  altitude  to  be  regular;  on  the  contrary,  they 
encounter  layers  of  air  throughout  which  the  temperature  is 
practically  unchanged.  They  also  encounter  other  layers  in 
which  an  inversion  occurs — that  is,  the  temperature  rises  with 
increasing  altitude.  In  other  words,  instead  of  a  uniform 
temperature  gradient  from  ground  level  to  stratosphere,  the 
air  consists  of  a  succession  of  layers,  differing  in  temperature, 
humidity  and  horizontal  velocity  of  movement.  Usually  the 
planes  of  contact  between  adjacent  layers  are  indicated  by 
clouds. 

The  air  of  adjacent  layers,  or  strata,  does  not  readily  mix  one 
with  the  other.  Smoke,  dust,  and  cloud  matter,  rising  to  the 
top,  or  sinking  to  the  bottom  of  a  layer  does  not  always  pene- 
trate the  adjacent  layer.  In  the  absence  of  strong  winds  such 
matter  is  apt  to  spread  out  laterally.  Moreover,  the  aviator, 
in  passing  from  one  layer  to  another,  is  apt  to  receive  a  sharp 
bump  at  the  plane  of  contact.  In  meteorology  the  plane  of 
contact  is  commonly  known  as  a  ceiling  or  lid. 

The  convectional  layer  of  air — that  is,  from  ground  level 
to  stratosphere — is  marked  by  constant  motion  as  noted,  the 
movements  consisting  of  general  circulation,  local  winds  and 
the  turbulence  connected  with  vertical  movements.  There  seems 
to  be  no  such  complexity  of  movement  in  the  stratosphere; 
indeed  the  knowledge  of  the  movements  of  the  air  in  the  strato- 
sphere is  next  to  nothing.  Tidal  movements  probably  warp 


58         THE  AIR:    MAJOR   CIRCULATION;    LOCAL  WINDS 

the  shape  of  the  shell  of  air  composing  it;  but  they  may  not 
cause  a  general  circulation.  The  fact  that  the  air  of  the  strato- 
sphere is  warmer  in  high  than  in  equatorial  latitudes  indicates 
that  a  circulation  of  some  sort  exists  and  that  the  general  move- 
ment may  be  the  reverse  of  that  of  the  lower  shell  of  air.  The 
coldest  air  is  over  the  warmest  zone. 

Winds  Encountered  by  the  Airman. — The  marine  pilot  is 
concerned  wholly  with  the  horizontal  movements  of  the  surface 
air;  he  is  not  conscious  of  the  updraughts  or  the  downdraughts 
of  convection.  To  the  airman,  on  the  other  hand,  the  hori- 
zontal air  movements  are  usually  less  of  consequence  than  the 
vertical  movements.  Good  air  for  flying  must  be  free  from 
holes  and  bumps. 

An  air  hole  is  not  a  vacuous  space,  nor  is  it  one  in  which 
the  density  of  the  air  is  abnormally  low.  Sometimes  it  is  a 
downdraught;  quite  often  it  is  convectionally  still  air.  If  the 
airman  has  been  flying  over  hot,  bare  ground,  where  the  up- 
draught  is  strong,  his  plane  takes  a  drop  when  he  passes  over  a 
patch  of  greensward,  where  the  updraught  ceases;  this  is  the 
airman's  "hole."  In  going  from  convectionally  still  air  into 
an  updraught,  he  gets  a  "bump."  The  same  result  is  apparent 
if,  while  traversing  a  downdraught,  he  strikes  still  air. 

It  has  been  noted  that  the  air  ranges  itself  in  layers  differing 
in  density,  temperature,  and  moisture  content.  In  many  cases 
an  acquired  sense  born  of  experience  enables  the  airman  to 
discern  these  layers  and  to  adjust  the  wings  of  his  plane  in 
encountering  them.  Frequently  a  sheet  of  smoke  or  dust 
separates  two  cloud  layers  and  experience  has  taught  how  to 
avoid  or  how  to  penetrate  it. 

Air  is  either  going  up  or  coming  down.  Turbulent  ascending 
currents  are  manifest  in  the  rapid  motion  of  cumulus  clouds, 
both  within  the  cloud  and  beneath  it.  Measurements  have 
shown  that  ordinary  updraughts  may  have  a  velocity  of  10 
feet  per  second;  under  a  cumulus  cloud  of  the  thunderhead 
type  the  velocity  may  be  as  high  as  40  feet  per  second.  The 
updraughts  that  produce  clouds  give  visible  signs  of  their 
existence.  Those  caused  during  clear  days  by  bosses  of  rock  or 
by  bare  ground  are  not  so  easily  detected.  They  are  not  apt 
to  begin  existence  until  the  sun  is  high  enough  to  heat  the  areas 
producing  them;  they  rarely  form  during  cloudy  days. 


WINDS  ENCOUNTERED  BY  THE  AIRMAN  59 

Downdraughts  are  sometimes  real  and  sometimes  only 
apparent.  In  passing  from  an  ascending  current  into  still  air 
the  drop  may  be  real,  but  the  downdraught  may  be  merely  ap- 
parent. In  flying  from  an  adverse  wind  into  a  wind  blowing  in 
the  direction  in  which  the  plane  is  moving,  the  drop  is  real  but 
the  downdraught  is  apparent  merely. 

There  are  actual  downdraughts,  however,  which  the  airman 
is  certain  to  encounter — because  air  going  up  must  be  balanced 
by  air  coming  down.  Just  as  water  pours  over  a  perpendicular 
ledge,  forming  thereby  a  cataract,  so  air  is  usually  pouring  over 
a  steep  scarp  in  a  similar  manner.  The  air  over  a  plateau  is 
apt  to  be  colder  than  that  several  hundred  feet  below.  More 
certainly  it  will  be  colder  if  it  has  traversed  great  fields  of  snow. 
When,  therefore,  it  reaches  a  steep  scarp  it  pours  over  the  edge 
by  virtue  of  its  own  gravity.  Air-falls  of  this  sort  are  common 
in  mountainous  regions,  but  they  rarely  occur  in  lowlands. 

Billow-cloud  levels  may  be  a  serious  problem  to  the  airman, 
not  because  they  interfere  with  visibility,  but  because  occasionally 
they  do  not  do  so.  When  billow  clouds  are  in  sight  the  airman 
may  fly  above  them  or  below  them.  If  the  two  wind  layers 
have  about  the  same  degree  of  humidity  there  may  be  no 
clouds  to  indicate  the  position  of  the  plane  of  contact.  Once 
within  this  plane,  the  airman  experiences  a  series  of  discon- 
certing bumps,  due  to  the  quick  transition  from  one  billow  to 
another;  sometimes  he  finds  it  difficult  to  rise  to  the  upper 
layer  of  air. 

Gusty  winds,  eddies  and  whirls  occur  most  frequently  near 
ground  level;  they  rarely  affect  high  flights.  Even  in  low 
flights  they  are  infrequent,  unless  the  plane  is  within  the  in- 
fluence of  cumulus  clouds.  They  are  disconcerting  in  making 
sharp  turns  and  they  may  be  dangerous  in  making  a  landing. 


CHAPTER  VII 

THE    MOISTURE    OF    THE    AIR  :    EVAPORATION    AND 
CONDENSATION 

Water  vapor  and  floating  dust  are  components  of  the  air 
which  vary  from  day  to  day  and  even  from  hour  to  hour.  All 
the  waters  of  the  land  are  derived  from  the  water  vapor  of 
the  air;  and  this  in  turn  is  brought  from  the  oceans.  Inas- 
much as  life  in  its  various  forms  depends  on  the  process  whereby 
ocean  waters  are  taken  into  the  air  and  are  dropped  upon  the 
land  as  rain  or  as  snow,  the  study  of  the  water  vapor  content 
of  the  air  is  of  vital  importance  to  humanity.  Before  the  waters 
of  the  sea  can  be  poured  over  the  land,  several  distinct  processes 
take  place:  Evaporation,  diffusion,  condensation,  and  precipitation. 

Evaporation. — It  is  assumed  that  the  molecules  of  a  volume 
of  water  are  in  constant  motion  among  themselves.  Some  of 
the  molecules  at  the  surface  are  in  such  rapid  motion  that 
they  bombard  themselves  into  the  air,  thereby  becoming  a  part 
of  it.  This  loss  to  the  water  goes  on  at  all  ordinary  tempera- 
tures and  even  at  very  low  temperatures.  At  212°  F  (100°  C) 
the  pressure,  or  tension  of  the  vapor  is  as  great  as  that  of  the 
air,  and  the  water  is  said  to  boil. 

In  meteorology,  evaporation  is  a  term  applied  practically 
to  the  net  loss  of  water,  or  other  liquid  exposed  to  the  air.  Free 
water  surfaces,  soil  and  vegetation  have  each  their  problems; 
meteorology  is  concerned  chiefly  with  evaporation  from  a  free 
surface  of  water.  Diffusion  of  the  water  vapor  derived  from  the 
ocean,  and  from  bodies  of  fresh  water,  is  so  universal  that  in 
no  part  of  the  earth  is  the  air  free  from  water  vapor. 

Various  conditions  affect  evaporation.  Under  ordinary 
conditions  of  light  winds  and  moderately  dry  air  the  rate  of 
evaporation  is  proportional  to  the  surface.1  It  is  also  directly 

1  In  still  air  over  a  circular  area  evaporation  increases  as  the  square  root 
of  the  area;  with  a  horizontal  wind  it  varies  approximately  in  theory,  at 
least,  as  the  three-fourth  power  of  the  area.  If  the  rate  be  calculated  in 
direct  proportion  to  the  area,  the  result  will  not  be  materially  incorrect. 

60 


CONDENSATION  61 

proportional  to  the  difference  in  the  readings  of  the  dry  bulb 
and  the  wet  bulb  of  a  sling  psychrometer.  Evaporation  in- 
creases very  rapidly  with  a  dry  wind  1  and  more  slowly  as  the 
relative  humidity  of  the  air  increases.  It  increases  rapidly 
with  rising  temperature  and  decreases  with  falling  temperature. 
It  increases  inversely  with  barometric  pressure.  The  rate  of 
evaporation  of  sea  water  is  about  95  per  cent  that  of  fresh 
water,  all  other  conditions  being  the  same. 

Condensation. — The  process  whereby  water  vapor  changes 
to  a  liquid  form  is  condensation.  Condensation  may  occur  as 
a  result  of  mechanical  processes,  such  as  pressure  and  artificial 
cooling;  in  the  free  air,  however,  it  results  from  cooling  by 
contact,  cooling  by  mixture,  or  cooling  by  expansion — prac- 
tically adiabatic  cooling.  More  definitely:  warm  air  resting 
on  the  ground,  or  on  the  sea,  may  be  cooled  by  contact  there- 
with, until  some  of  its  moisture  is  condensed.  An  area  of 
warm,  moist  air  may  be  invaded  by  a  cold  wind  and  the 
mixing  process  may  cool  the  vapor  to  the  temperature  of  con- 
densation. A  body  of  air  warmed  above  the  temperature  of 
the  surrounding  air  is  pushed  upward.  Its  expansion  causes 
adiabatic  cooling  and  if  the  temperature  falls  below  that  of 
saturation,  condensation  of  the  water  vapor  occurs.  Prac- 
tically all  the  cases  of  condensation  with  which  weather  science 
has  to  do  result  from  one  or  another  of  the  causes  named.  The 
condensation  resulting  from  contact  causes  dew  and  frost; 
that  which  results  from  mixing  causes  fog  and  cloud;  that 
resulting  from  adiabatic  cooling — that  is,  updraught — causes 
rain  and  snow.  There  are  occasional  exceptions  to  the  fore- 
going, especially  where  superficial  turbulence  of  the  air  is  in- 
volved; in  the  main,  however,  these  processes  of  condensation 
are  fundamental  in  weather  science. 

Dust  Motes  and  Condensation. — The  invisible,  floating  dust 
motes  of  the  air  and  many  of  the  gaseous  products  of  com- 
bustion are  important  factors  in  condensation.  Each  droplet 
of  cloud  or  fog  condenses  upon  a  dust  mote  or  upon  a  hygro- 
scopic gas  product.2  In  general,  the  dust  motes  which  cool  most 

1  The  rate  varies  approximately  as  the  square  root  of  the  wind  velocity, 
and  as  the  cube  of  the  square  root  of  the  diameter  of  a  circular  container. 

2  There  are  certain  cases  of  super-saturation  to  which   this  statement 
is  an  exception;    indeed,  condensation  is  still  a  field  for  investigation. 


62  THE    MOISTURE    OF    THE    AIR:     EVAPORATION 

quickly  are  regarded  as  the  "most  favorable"  nuclei.  Were  it 
not  for  this  feature  of  condensation,  gentle  rains  would  become 
sporadic  cloudbursts.  The  measurement  of  the  dust  content  of 
the  air  is  not  yet  a  part  of  the  scope  of  weather  observations, 
but  the  importance  of  it  is  universally  recognized. 

Conditions  of  Condensation  and  Precipitation. — In  an- 
other chapter  the  relation  of  temperature  to  the  amount  of 
water  vapor  has  been  discussed.  The  absolute  water  vapor 
content  of  the  air  is  the  gross  amount  of  water  it  contains.  This 
is  usually  estimated  in  grains  per  cubic  foot  or  in  milligrams 
per  cubic  decimeter.  Between  the  twenty-fifth  and  fiftieth 
parallels  of  latitude  the  amount  of  water  per  cubic  foot  averages 
roughly  from  I  to  3  grains  in  winter  and  from  5  to  7  grains  in 
summer — north  to  south.  The  proportion  varies,  however. 
Sea  winds  are  wet  winds;  land  winds  are  usually  dry.  The 
higher  the  temperature,  the  greater  the  possible  absolute  con- 
tent of  water  vapor. 

Condensation  does  not  begin  until  the  temperature  of  the 
air  has  reached  the  degree  below  which  only  a  certain  proportion 
of  vapor  can  exist — that  is,  below  the  temperature  of  saturation, 
or  dew-point.  Any  excess  is  condensed  and  appears  in  one  or 
another  of  the  forms  noted. 

Relative  Humidity. — The  water  vapor  content  of  the  air 
which  is  not  condensed  is  so  important  to  life  and  to  human 
comfort  that  its  measurement  is  an  essential  part  of  Weather 
Bureau  observations.  The  higher  the  temperature  of  the  -air, 
the  greater  the  amount  of  water  vapor  it  may  contain — about 
4  times  as  much  at  70°  F  (21°  C)  as  at  32°  F  (o°  C),  and  10 
times  as  much  at  100°  F  (38°  C) ;  hence  the  term  relative  hu- 
midity. This  is  expressed  in  terms  of  the  per  cent  of  water  vapor 
necessary  to  saturation.  Thus,  if  the  relative  humidity  is  50 
per  cent,  half  the  vapor  necessary  for  saturation  at  the  ob- 
served temperature  is  present. 

Ordinarily,  the  humidity  is  highest  in  early  morning,  when 
the  temperature  is  lowest;  it  is  usually  lowest  at  the  warmest 
part  of  the  day.  On  dewy  and  frosty  mornings  the  humidity  at 
ground  level  is  100  per  cent;  a  few  feet  above  ground  it  is 
probably  at  96  per  cent;  during  the  hottest  part  of  the  day 
it  may  be  as  low  as  30  per  cent,  or  even  lower;  on  cloudy  days 
it  may  not  vary  materially  during  the  day.  During  foggy 


RELATIVE  HUMIDITY  63 

weather  it  is  practically  100  per  cent.1  During  summer 
rainstorms  it  is  approximately  95  per  cent. 

The  relative  humidity  of  the  air  has  a  profound  effect  upon 
public  health.  General  Greely  noted  the  fact  that,  during 
prolonged  spells  of  very  dry  air  when  the  per  cent  of  humidity 
fell  materially  below  the  normal,  a  notable  increase  in  the 
death  rate  followed.  Dr.  Ellsworth  Huntington  has  shown 
that  the  same  result  is  true  of  the  death  rate  in  hospitals. 

Humanity,  both  the  conscious  and  the  sub-conscious  self, 
is  sensitive  to  changes  in  temperature,  noting  a  difference  even 
of  i  degree  Fahrenheit.  The  conscious  self  rarely  notices  changes 
in  humidity  between  35  per  cent  and  85  per  cent.  The  sub- 
conscious self  is  far  more  sensitive;  it  rebels  against  a  condition 
of  humidity  materially  higher  than  75  per  cent  or  lower  than 
40  per  cent  when  the  temperature  of  the  air  is  that  of  comfort. 
There  is  a  noticeable  difference  to  the  feelings  between  indoor 
and  out-of-door  air.  Indoors,  a  humidity  of  25  per  cent  is 
extremely  uncomfortable;  out  of  doors  it  is  hardly  perceptible. 

During  the  winter  season  when  buildings  are  artificially 
heated,  the  humidity  of  living-rooms  is  not  often  above  40 
per  cent;  usually  it  is  lower  than  35  per  cent;  in  school  rooms 
it  may  be  less  than  25  per  cent.  Dr.  C.-E.  A.  Winslow  has 
pointed  out  the  effect  upon  the  health  of  the  pupils  of  air  so 
deficient  in  moisture.2  P.  R.  Jameson,  using  empiric  but  very 
practical  standards  of  measurement — that  is,  comfort  or  dis- 
comfort— has  tabulated  the  results  of  several  thousand  tests: 

very  cold 
Rel.  Hum.  75%  \   65°  F  chiliy 

comfortable 
very  cold 
Rel.  Hum.  50%  \    50°  F  chilly 

comfortable 
very  cold 
Rel.  Hum.  30%  \   65°  F  chilly 

comfortable 

1  During  the  prevalence  of  a  "dry  fog"  the  humidity  may  be  not  higher 
than  85  per  cent. 

2  Dr.  Winslow  has  noted  that  the  air  of  schoolrooms  in  winter  is  as  dry 
as  that  of  a  desert.     As  a  matter  of  fact,  the  air  of  the  Gila  Desert,  Arizona, 
is  rarely  so  dry  as  that  of  a  schoolroom  at  9  o'clock, 


64  THE    MOISTURE    OF    THE    AIR:     EVAPORATION 

That  is,  with  the  relative  humidity  at  50  per  cent,  the 
temperature  of  comfort  is  10  degrees  lower  than  with  a  very  dry 
or  a  moist  air.  These  conclusions  do  not  differ  from  those  of 
Dr.  Huntington. 

Many  manufacturers  have  installed  humidifiers  within 
their  factories  in  order  to  provide  wholesome  air  to  their  em- 
ployees and  a  correct  atmosphere  for  the  economical  produc- 
tion of  their  output.  Exhaust  ducts  carry  the  air  from  the 
work  rooms  to  the  humidifier  where  it  is  screened,  washed,  and 
returned  to  the  various  rooms  with  but  little  loss  of  temperature. 
The  saving  in  fuel  very  soon  pays  the  cost  of  a  humidifying 
plant. 

Forms  of  Condensation. — The  condensation  of  the  water 
vapor  of  the  air  takes  place  in  many  forms — fog,  cloud,  dew, 
frost,  rain,  snow,  and  hail.  The  ''sweating"  of  walls,  and  the 
film  of  moisture  that  forms  on  the  outside  of  a  vessel  filled  with 
iced  water  are  also  examples  of  condensation.  In  any  case 
the  cause  is  the  same;  the  temperature  of  the  air  falls  below 
the  temperature  of  saturation  and  the  excess  of  water  vapor 
is  condensed  in  one  or  another  of  the  forms  noted.  The  forma- 
tion of  fog  and  cloud  are  considered  in  the  following  chapter; 
hail  is  a  feature  of  thunder-storms. 

Dew. — Dew  consists  of  the  moisture  condensed  on  such 
surfaces  as  radiate  their  warmth  after  sundown.  If  the  chilling 
of  the  air  next  to  such  surfaces  carries  its  temperature  below 
that  of  saturation — that  is,  the  "dew-point" — the  excess  is 
deposited  in  the  form  of  minute  droplets.  Not  infrequently  so 
much  moisture  is  deposited  that  foliage  and  grass  become  very 
wet.  Vegetation  radiates  its  heat  rapidly,  and  therefore  dew  is 
apt  to  form  copiously  thereon.  At  night  the  temperature  of  the 
air  two  or  three  inches  from  the  ground  may  be  as  much  as  5 
degrees  lower  than  at  a  height  of  6  feet.  Dew  therefore  may 
form  on  grassy  surfaces  when  none  occurs  on  objects  materially 
above  ground. 

Falling  temperature  at  night  is  the  rule;  nevertheless,  dew 
does  not  always  form.  The  temperature  may  not  go  down  to 
the  dew-point;  the  absolute  humidity  may  be  very  low;  wind 
may  keep  the  air  stirring  so  that  the  air  next  the  ground  may 
not  remain  long  enough  to  be  cooled  to  the  dew-point;  low 
clouds  may  prevent  the  radiation  of  ground  warmth;  a  "lid" 


FROST  65 

also  may  prevent  radiation.  For  the  foregoing  reasons  the  prob- 
lems concerning  the  formation  of  dew  are  of  much  importance. 

Frost. — If  the  temperature  of  the  air  is  below  freezing,  the 
water  vapor  will  be  deposited  in  the  form  of  minute  crystals 
of  ice,  which  reflect  the  light  in  such  a  manner  that  a  silvery 
appearance  results — the  hoar  frost  of  popular  tradition.  In 
some  instances  the  moisture  is  doubtless  deposited  as  dew, 
which  afterwards  is  frozen.  Sometimes,  too,  partly  melted 
frost  or  slowly  freezing  dew  forms  a  glazed  and  semi-trans- 
parent coating — the  rime  of  tradition.  From  the  nature  of  the 
case,  rime  is  more  hurtful  to  vegetation  than  is  hoar  frost. 

When  rain  freezes  as  it  falls  on  leaves,  stalks,  and  twigs  the 
ice  varnish  is  the  traditional  black  frost.  Strictly  speaking, 
it  is  not  frost  at  all.  It  is  a  freezing  which  involves  the  surface 
of  the  vegetation.  The  superficial  juices  of  the  plant  are  frozen, 
to  the  extent  that  the  cells  of  the  plant  are  ruptured. 

Hoar  frost  injures  tender  plants  but  does  not  necessarily 
kill  them.  Black  frost,  on  the  other  hand,  is  apt  to  kill  tender 
plants  and  to  injure  many  hardy  plants.  A  temperature  as  low 
as  25°,  without  frost,  may  be  as  fatal  to  tender  plants  as  a  black 
frost. 

Warnings  of  late  spring  and  early  fall  frosts  are  sent  out 
from  Weather  Bureau  stations.  Close  observation,  however, 
will  enable  one  to  foretell  a  possible  frost  by  watching  the 
temperature  and  humidity.  When  the  air  is  still,  the  humidity 
high,  and  the  sky  clear,  frost  may  be  expected  if  the  temperature 
at  sunset  is  40°  or  lower;  indeed,  under  such  conditions  the 
temperature  is  likely  to  fall  to  the  freezing  point  by  2  o'clock  on 
the  following  morning  and  to  remain  below  freezing  for  a  short 
time  after  sunrise. 

The  greater  likelihood  of  frost  in  low  spots,  such  as  valley 
floors,  as  compared  with  the  higher  levels  of  the  adjacent 
slopes,  is  an  important  factor  in  fruit  farming  and,  in  fruit-grow- 
ing regions,  pretty  accurate  surveys  have  been  made  of  the  lands 
likely  to  be  visited  by  killing  frosts.  Nevertheless,  by  far  the 
greater  area  of  tender  crops  is  within  the  region  of  killing  frosts; 
hence  the  necessity  of  making  use  of  all  available  knowledge 
in  the  matter.1 

1  Bulletin  V  of  the  U.  S.  Weather  Bureau  publications  is  a  summary  of 
observations  collected  from  more  than  one  thousand  stations  and  sub- 


66 


THE    MOISTURE    OF    THE    AIR:     EVAPORATION 


j-\     H  n&-— 1»\    \ 

4-Vv._j  i       3">  m  f-J \ 1 

»  5fS'^ — ioo  ^    5   gS      1 1 

*oo|r~|c\'  o'oooo  ^co  \ 

\     1-1  .'  -*      -«  OOj'-'JO'^OO  '"'-•_        01 

\  «     -*  oo   o  a   -i   ! 


FROST  67 

Weather  Bureau  records  make  a  distinction  between  light 
frosts  and  killing  frosts,  the  latter  being  so  called  because  of  their 
destructive  effects.  Ground  frosts,  as  a  rule,  are  not  killing; 
the  freezing  temperature  does  not  extend  more  than  a  few 
inches  above  the  grass.  If  the  freezing  temperature  extends 
so  high  that  frost  covers  the  roofs  of  buildings,  the  frost  is  apt 
to  be  killing.  Records  of  the  dates  of  the  latest  killing  spring 
frost  and  the  earliest  killing  fall  frost  are  highly  important  from 
the  fact  that  the  number  of  days  intervening  constitutes  the 
growing  season. 

In  the  latitude  of  the  Great  Lakes  the  growing  season  is 
frofn  no  days  to  150  days;  in  the  latitude  of  Illinois  and 
Missouri  it  is  from  150  days  to  200  days;  in  the  belt  extending 
from  the  northern  boundary  of  Tennessee  to  the  Gulf  it  is  from 
200  to  300  days.  Florida  and  Texas,  south  of  the  twenty- 
seventh  parallel,  are  very  rarely  visited  by  killing  frosts. 

stations.  The  information  is  graphically  charted  on  maps  which  show  the 
dates  of  late  spring  and  early  fall  frosts,  the  average  dates  of  killing  frosts, 
and  the  number  of  days  between  spring  and  fall  frosts.  The  following 
paragraphs  apply  pretty  generally  to  all  parts  of  the  United  States: 

Frost  becomes  more  severe  as  one  goes  from  hillside  to  low  spots,  such 
as  hollows  and  stream  valleys. 

It  is  more  severe  on  the  grass  than  at  shrub  heights.  It  may  form  on 
the  grass  when  the  temperature  3  or  4  feet  above  ground  is  several  degrees 
above  the  freezing-point. 

If  the  temperature  at  sunset  is  not  lower  than  40°  F  (5°  C)  and  the  sky 
is  overcast,  frost  is  not  likely  to  occur.  But  if  the  sky  is  clear  and  the  wind 
is  at  calm,  frost  is  likely. 

With  a  brisk  wind  and  a  sky  either  clear  or  cloudy,  frost  is  not  likely 
to  occur  unless  the  temperature  falls  materially  below  freezing. 

If  the  air  is  moist  at  sunset  and  the  temperature  is  40°  F  or  lower,  frost 
is  likely  to  occur  even  with  a  light  wind;  but  if  fog  occurs,  enough  latent 
heat  may  be  set  free  to  prevent  frost.  A  low-lying  fog  is  a  blanket  which 
retards  radiation,  not  only  from  grass  and  shrubbery,  but  from  the  ground 
itself. 


CHAPTER  VIII 
THE  MOISTURE  OF  THE  AIR:    FOG  AND  CLOUD 

FOG 

In  his  "Floating  Matter  of  the  Air"  Tyndall  demonstrated 
that,  when  the  air  pressure  under  the  receiver  of  an  air  pump 
was  reduced,  the  cooling  of  the  air  by  expansion  produced  a 
perceptible  fog.  He  demonstrated  also  that,  if  the  air  admitted 
to  the  receiver  were  filtered,  a  second  exhaustion  would  produce 
a  fog  only  to  an  extent  scarcely  observable,  or  not  at  all.  In 
other  words,  the  dust  motes  and  molecules  of  hygroscopic 
gases  are  necessary  for  condensation.  When  there  were  no 
longer  any  dust  motes,  there  was  no  condensation. 

Fog  and  cloud  are  the  most  striking  examples  of  condensa- 
tion on  a  large  scale;  in  weather  science  it  is  commonly  called 
volume  condensation.  One  cannot  readily  make  a  distinction 
between  fog  and  cloud;  in  general,  fog  is  cloud  on  the  ground, 
while  cloud  is  fog  high  in  the  air.  When  the  blue  sky  be- 
comes white,  the  change  in  color  is  due  to  condensation — 
perhaps  water  dust,  perhaps  ice  dust.  If  the  condensation 
thickens,  distant  objects  become  blurred  by  the  accumulated 
condensation;  a  moisture-haze,  quite  distinct  in  color  from  the 
blue  dust-haze,  occurs.  Perhaps  the  white  sky  might  not  be 
called  cloud;  but  if  the  condensation  increases  until  the  color 
becomes  a  dark  gray,  by  common  consent  it  is  "cloud"  in  the 
air,  or  "fog"  if  it  extends  to  the  ground.  The  distinction  is 
merely  one  of  degree.  Fog  and  cloud  are  examples  of  condensa- 
tion; but  until  the  droplets  coalesce  into  drops  that  fall  to  the 
ground  they  are  not  precipitation. 

It  is  likely  that  fog  and  cloud  droplets  vary  much  in  size; 
but  definite  knowledge  of  the  extent  of  this  variation  is  wanting. 
Wells  found  that  fog  droplets  were  approximately  0.0002  inch 
(0.005  mm)  in  dimension,  and  that  the  fog  droplets  in  a  cubic 
yard  were  not  far  from  7  grains  in  weight. 

68 


FOG  TYPES  69 

Fog  Types. — A  common  illustration  of  fog  formation  may 
be  observed  when  a  cake  of  ice  is  at  the  doorstep.  Almost  im- 
mediately it  begins  to  "steam."  The  ice  chills  the  air  in  con- 
tact below  the  dew-point,  and  condensation  is  at  once  ap- 
parent in  the  form  of  fog.  Condensation  liberates  enough  latent 
heat  to  give  the  moisture  a  certain  amount  of  updraught,  and 
therefore  a  steaming  effect.  It  is  an  instructive  illustration  of 
contact  cooling,  and  the  fog  produced  is  the  radiation  fog  of 
weather  science. 

On  still  nights  during  spring  and  fall,  fog  is  frequent  over 
rivers  and  ponds,  especially  in  relatively  low  places.  If  the  air 
is  still  over  such  bodies  of  water  during  the  day,  it  is  apt  to  be 
moist.  Therefore  the  normal  lowering  of  temperature  soon 
reaches  the  dew-point  and,  as  a  result,  a  radiation  fog  forms. 
Sometimes  its  depth  is  only  a  few  feet ;  occasionally  it  overtops 
buildings  and  trees. 

In  various  instances  fogs  hover  over  manufacturing  districts 
when  nearby  rural  areas  are  free  from  them.  It  is  pretty  certain 
that  the  products  of  combustion  are  the  "favorable  nuclei"  in 
such  cases.  Dr.  Owen  of  the  British  Meteorological  Office 
found  that  many  such  floating  particles  were  extremely  hygro- 
scopic, and  that  they  tended  to  produce  condensation  when  it 
did  not  occur  in  air  free  from  them.1  At  all  events,  the  city  fog 
has  become  a  factor  in  meteorology  as  well  as  in  city  traffic. 

Advection  fog2  is  the  name  given  to  fogs  that  result  when 
warm  moist  air  invades  a  surface  so  cold  that  dew-point  tem- 
perature is  reached.  The  sea  fogs  of  the  North  Atlantic  are  an 
example.  Warm,  moist  winds  of  a  southerly  origin  invade  the 
region  of  cold  Arctic  currents,  and  condensation  of  the  moisture 
brought  to  the  region  occurs.  "Skin  friction"  between  wind  and 
water  causes  the  eddying  movements  of  the  air  known  as 
turbulence,  and  the  fog  blanket  extends  higher  and  higher  as 

1  In  the  fog  over  a  manufacturing  district  Dr.  Owen  also  found   moisture 
droplets  coated  with  liquid  hydrocarbon,  derived  evidently  from  coal  smoke. 
In  other  words,  the  fog  droplet  itself  was  a  nucleus  upon  which  the  smoke- 
hydrocarbon  condensed.     The  author  failed  to  find  this  condensation  in  the 
manufacturing  districts  near  New  York  City;    but  it  is  highly  probable  that 
it  occurs  in  such  atmospheres  as  those  of  Pittsburgh  and  South  Chicago. 

2  From  the  Latin  ad,  "to,"  and  vehere,  to  "carry"— that  is,  fog  produced 
by  conditions  carried  to  a  locality  from  an  external  source.      Although  the 
name  is  comparatively  recent,  it  is  very  aptly  formed. 


70       THE    MOISTURE    OF    THE    AIR:     FOG  AND   CLOUD 


CLOUD  PHOTOGRAPHY 

According  to  Arthur  J.  Weed,  Chief  Instrument  Maker,  U.  S.  Weather 
Bureau,  the  first  requisite  for  cloud  photography  is  a  good  camera  with  a 
rigid  support.  To  this  equipment  a  ray  filter  to  shut  out  the  excess  of  actinic 
rays  from  the  blue  sky  is  added.  The  filter,  consisting  of  colored  screens  vary- 


Ellerman,  photo, 
Nimbus,  with  fog  or  stratus  hovering  in  the  valleys,  Mount  Wilson,  Cal. 

ing  from  yellow  to  red,  is  usually  necessary,  inasmuch  as  the  exposure  required 
for  the  cloud  results  in  an  over  exposed  sky.  Very  dense  clouds  may  be  photo- 
graphed without  the  use  of  a  ray  filter.  Cirrus  clouds,  however,  require  a 
strongly-colored  ray  filter.  A  black  mirror  answers  the  purpose  of  a  ray 
filter  and,  in  certain  cases,  gives  a  better  negative.  The  details  of  cloud 
photography  are  described  in  the  Monthly  Weather  Review,  August,  1920. 


CLOUDS  71 

the  chilling  of  the  air  progresses.  Advection  fogs  are  more  apt 
to  follow  gentle  movements  of  the  air;  gale  winds  may  create 
mixing  to  the  extent  that  dew-point  temperature  is  not  reached. 
The  advance  of  a  cold  wave  moving  gently  into  a  region  of  warm, 
moist  air — the  fog  in  front  of  a  high — is  an  example  of  ad- 
vection  fog. 

Veto  cloud  l  is  a  name  now  commonly  given  to  fog  drifting 
in  from  the  sea  and  hovering  over  a  coast  a  few  hundred  feet 
from  the  ground.  It  is  of  frequent  occurrence  along  the  coast 
of  southern  California  during  summer  months,  and  is  oc- 
casional along  the  Atlantic  coast.  The  velo  is  an  example  of 
advection  condensation.  Perhaps,  strictly  speaking,  it  should 
be  classed  as  cloud  rather  than  as  fog;  nevertheless  it  is  ad- 
vective  condensation.  Inasmuch  as  the  term  "high  fog"  is 
sometimes  popularly  used  to  denote  a  very  thick  fog  meteor- 
ologists have  generally  adopted  the  term  'Velo."  The  velo  is 
rarely  more  than  1000  feet  high. 

CLOUDS 

Cooperative  observers  are  not  required  to  report  information 
concerning  cloudiness,  except  the  extent  of  cloud-covered  sky 
during  the  daylight  period.  At  the  regular  Weather  Bureau 
stations  the  character,  movement  and  height  of-  clouds  are 
recorded  and  at  some  stations. nephoscopes  are  provided.  With 
the  aid  of  these  instruments,  the  velocity  of  the,  clouds,  and 
therefore  that  of  the  upper  winds,  may  be  determined. 

The  photogrammeter  is  one  of  the  most  practical  instru- 
ments for  measuring  cloud  heights.  It  consists  of  a  pair  of 
cameras  mounted  in  the  same  manner  as  a  surveyor's  transit. 
Two  instruments  set  at  different  positions  are  employed.  The 
sensitive  plates  are  ruled  with  intersecting  horizontal  and  vertical 
lines.  By  the  aid  of  these,  the  photographs  of  the  cloud  indicate 
its  comparative  position,  and  from  this  both  its  altitude  and  its 
velocity  may  be  determined.  Air  navigation  now  demands 
definite  knowledge  of  wind  at  different  elevations,  and  this 
knowledge  is  best  obtained  by  a  study  of  the  clouds. 

Formation  of  Clouds. — A  cloud  consists  of  an  aggregation 

1  From  a  Spanish  word  meaning  "veil."  The  velo  is  a  characteristic 
of  San  Diego. 


72       THE    MOISTURE    OF    THE    AIR:     FOG   AND    CLOUD 


I 


CLOUD  CLASSIFICATION  73 

of  visible  particles  of  condensed  water  vapor.  As  in  the  for- 
mation of  fog,  each  particle  of  cloud  matter  has  condensed 
upon  a  dust  mote.  One  cannot  say  why  cloud  matter  floats  in 
the  air,  apparently  contrary  to  the  laws  of  gravity.  A  theory 
that  the  cloud  particle  is  repelled  from  the  earth  because  it  is 
charged  with  the  same  kind  of  electricity  has  been  advanced; 
but  it  is  not  certain  that  this  theory  satisfies  all  conditions. 
That  clouds  form  and  disappear  in  accordance  with  the  laws 
of  temperature  and  dew-point  is  the  fact  that  is  important  in 
weather  science. 

For  convenience,  cloud  matter  may  be  considered  to  be  in 
a  stage  of  condensation  intermediate  between  vapor  and  liquid 
— a  condition  which  may  be  brought  about  by  several  means: 

Local  ascending  currents,  or  updraughts,  which  are  vertical 
or  nearly  vertical; 

Very  slow  obliquely  ascending  currents; 

The  rapid  chilling  of  the  lower  air  by  the  radiation  of  earth 
warmth ; 

The  contact  of  high  air  layers  which  differ  in  temperature 
and  humidity. 

Any  one  of  the  foregoing  conditions  will  produce  cloud  if 
the  temperature  falls  below  the  dewpoint;  nevertheless  it  is 
probable  that  cloud  condensation  is  more  complex  in  fact 
than  the  foregoing  paragraphs  indicate. 

Classification. — Various  schemes  of  cloud  classification  have 
appeared  from  time  to  time.  Some  of  them  have  possessed 
great  merit,  but  have  been  too  complicated  for  practical 
use.  More  than  a  century  ago,  Luke  Howard,  of  London, 
devised  the  classification  upon  which  the  scheme  now  in  use 
was  elaborated  by  the  Cloud  Committee  of  the  International 
Meteorological  Congress  in  1891.  The  four  fundamental  forms 
are  cirrus,  cumulus,  stratus,  and  nimbus.1  Other  forms  are 
designated  by  the  combination  of  the  foregoing  terms.  Two 

1  From  the  Latin  cirrus  (pi.  cirri),  a  curl  or  wisp;  cumulus  (pi.  cumuli),  a 
heap,  or  pile;  stratus  (pi.  strati,  rarely  used),  a  layer;  nimbus  (pi.  not  used), 
a  rain  cloud.  The  adjective  derivatives  are:  cirro-,  cumulo-,  and  strata-. 
Other  definitive  adjectives  are  alto-,  high,  and  fracto-,  broken. 

1  Some  observers  still  employ  the  abbreviations  of  cloud  names  employed 
when  the  Weather  Bureau  was  a  part  of  the  Signal  Corps:  Cirrus,  C;  Cumu- 
lus, K;  Stratus,  S;  Nimbus,  N.  These  symbols  are  used  in  Army  and  Navy 
practice. 


74       THE    MOISTURE    OF    THE    AIR:     FOG    AND    CLOUD 


CLOUD  CLASSIFICATION  75 

factors,  appearance  and  altitude,  aid  the  observer  in  determin- 
ing the  name  and  character  of  a  cloud.  For  all  practical  pur- 
poses the  physical  form  and  appearance  must  always  be  the 
chief  feature  in  cloud  determination;  experience  will  teach  the 
observer  to  determine  whether  the  cloud  in  question  is  to  be 
classed  as  "upper,"  "intermediate,"  or  "lower";  a  distinction 
which  is  sometimes  essential.  The  following  is  the  classifica- 
tion elaborated  by  Abercromby  and  Hildebrandsson  and 
adopted  by  the  International  Meteorological  Congress: 

(a)  Detached  clouds  with  rounded  upper  outlines. 

(b)  Clouds  of  great  horizontal  extent  suggesting  a  layer  or  sheet. 

The  first  (a)  are  most  frequent  in  fair  weather;    the  second  (b)  are  wet- 
weather  clouds. 

{  a.   i.   Cirrus  1 

Upper  clouds,  30,000  feet  (oooo  meters) <   .  ~. 

I  b.  2.  Cirro-stratus 


Intermediate  clouds,  10,000  to  23,000  feet  (3000  to  [        3  „ 

v  \  a.  4.  Alto-cumulus 

7000  meters) 

(  b.  5.  Alto-stratus 

Lower  clouds,  less  than  6500  feet  (2000  meters) .  .    .  /  °"  6'  Strato-cumulus 

I  b.  7.  Nimbus 
(  top  6000  feet    (1800   meters);   r 

~.      ,      ,  ,.         ,1       base  4500  feet  (1400  meters)   I   «•  8-  Cumulus 
Clouds  of  diurnal 

ascending  currents    (  toP  10,000  to  260,00  feet  (3000 
]       to  8000  meters);  base  4500 

feet  (1400  meters) [*!•?•  Cumulo-nimbus 

High  fogs,  less  than  3500  feet  (1000  meters) co.  Stratus 

i.  CIRRUS  (Ci).1 — Detached  clouds  of  delicate  and  fibrous  appearance,  often 
showing  a  featherlike  structure,  generally  of  a  whitish  color.  Cirrus  clouds 
take  the  most  varied  shapes,  such  as  isolated  tufts,  thin  filaments  on  a  blue 
sky,  threads  spreading  out  in  the  form  of  feathers,  curved  filaments  ending 
in  tufts,  sometimes  called  cirrus  uncinus,  etc.;  they  are  sometimes  arranged 
in  parallel  belts  which  cross  a  portion  of  the  sky  in  a  great  circle  and,  by 
an  effect  of  perspective,  appear  to  converge  toward  a  point  on  the  horizon, 
or,  if  sufficiently  extended,  towards  the  opposite  point  also  (Ci-St  and  Cu-Ci, 
etc.,  are  also  sometimes  arranged  in  similar  bands). 

Cirrus  clouds  moving  from  the  southwest  indicate  falling 
temperature;  moving  from  the  northwest  they  indicate  the 
probability  of  rising  temperature.  They  are  the  mares'  tails 
and  cattails  of  sailors'  cant.  Near  the  horizon,  cirrus  clouds 
may  have  a  stratiform  appearance. 

1  For  the  sake  of  uniformity  of  definition  and  description,  the  following 
paragraphs  are  taken  from  the  report  of  the  Committee. 


76       THE    MOISTURE   OF    THE   AIR:     FOG   AND    CLOUD 


CLOUD  CLASSIFICATION  77 

2.  CIRRO-STRATUS    (Ci-St). — A    thin,  whitish  sheet  of  cloud,1   sometimes 
covering  the  sky  and  giving  it  only  a  milky  appearance;    it  is  then  called 
cirro-nebula — at  other  times  presenting  more  or  less  distinctly  a  formation 
like  a  tangled  web.     This  sheet  often  produces  halos  around  the  sun  and 
the  moon. 

This  name  is  apt  to  be  misleading  to  observers  who  have 
followed  the  old  nomenclature.  It  applies  not  so  much  to  the 
striated  or  banded  cirri  as  to  the  whitish,  or  creamy,  haze  with 
banded  or  feathery  edges.  Frequently  it  appears  as  a  whitish 
bank,  with  here  and  there  a  web  of  tangled  fibers;  at  times  it 
covers  the  whole  visible  sky.  The  halo  produced  when  a  cirro- 
stratus  film  is  in  front -of  the  moon  is  varied  in  form.  Oc- 
casionally mock  moons,  paraselenae,  are  formed ;  so  also  are  the 
light  pillar  and  the  "heavenly  cross." 

Cirro-stratus  clouds  have  long  been  associated  with  ap- 
proaching stormy  weather,  and  tradition  seems  to  be  borne  out 
by  investigation.  The  name  cirrus  haze  is  sometimes  applied 
to  cirro-nebula. 

3.  CIRRO-CUMULUS  (Ci-Cu). — MACKEREL  SKY. — Small  globular  masses  or 
white  flakes  without  shadows,  or  showing  very  light  shadows,  arranged  in  groups 
and  often  in  lines. 

Cirro-cumulus  clouds  are  not  always  distinguishable  from 
alto-cumulus  clouds.  They  are  much  higher,  however,  and 
the  arrangement  usually  possesses  a  geometric  regularity. 
C.  F.  Brooks  describes  them  as  "small  white  flakes  or  tenuous 
globular  masses  which  produce  no  diffraction  colors  when 
covering  the  sun  or  the  moon." 

4.  ALTO-STRATUS  (A-St). — A  thick  sheet  of  gray  or  bluish  color,  sometimes 
forming  a  compact  mass  of  dark  gray  color  and  fibrous  structure.     At  other 
times  the  sheet  is  thin,  resembling  thick  Ci-St;    and  through  it  the  sun  or 
the  moon  may  be  seen  dimly,  gleaming  as  through  ground-glass.     This  form 
exhibits  all  the  changes  peculiar  to  Ci-St,  but  it  is  about  one-half  as  high. 

It  is  not  always  easy  to  distinguish  alto-stratus  from  cirro- 
stratus  clouds.  One  cannot  always  estimate  its  altitude  and, 
if  the  cloud  is  thin,  it  may  be  about  as  white  as  a  cirro-stratus 
formation.  The  lower  edge  may  be  undulate,  but  it  is  hardly 

1  Not  every  "thin  whitish  sheet  of  cloud"  is  a  cirro-stratus  formation. 
The  low,  white  cloud  veil  of  winter  days  may  produce  a  halo;  but  it  is  not 
a  cirro-stratus  cloud. 


78       THE    MOISTURE    OF    THE    AIR:     FOG    AND    CLOUD 


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CLOUD  CLASSIFICATION  79 

mammillate  in  the  manner  of  mammato-cumulus  clouds.  The 
fibrous  alto-stratus  is  composed  of  snow  crystals.  It  does  not 
cause  halos.  The  compact  form  is  composed  of  water  droplets 
and  may  cause  coronas.  Alto-stratus  clouds  indicate  varying 
conditions  of  moisture  and  quiet  air,  rather  than  definite  weather 
conditions..  Nevertheless  rain  and  snow  may  fall  from  them.1 

5.  ALTO-CUMULUS  (A-Cu). — Large  globular  masses,  white  or  grayish,  partly 
shaded,  in  groups  or  lines,  and  often  so  closely  packed  that  their  edges  appear 
confused.     The   detached    masses   are   generally   larger   and    more   compact 
(resembling  St-Cu)  at  the  center  of  the  group,  but  the  thickness  of  the  layer 
varies.     At  times  the  masses  spread  themselves  out  and  assume  the  appear- 
ance of  small  waves,  or  thin,  slightly  curved  plates.     At  the  margin  they 
form  into  finer  flakes  (resembling  Ci-Cu).     They  often  spread  themselves  out 
in  lines  in  one  or  two  directions. 

It  is  evident  that  the  observer  will  record  alto-cumulus  as 
cirro-cumulus  and  vice  versa;  many  times  a  description  of  either 
will  fit  the  other.  Fortunately  such  an  error  is  harmless. 

6.  STRATO-CUMULUS  (St-Cu). — Large  globular  masses  or  rolls  of  dark  clouds 
often  covering  the  whole  sky,  especially  in  winter.     Generally  St-Cu  presents 
the  appearance  of  a  gray  layer  irregularly  broken  up  into  masses  of  which 
the  edge  is  often  formed  of  smaller  masses,  often  of  wavy  appearance  resembling 
A-Gu.    ^Sometimes  this  cloud-form  presents  the  characteristic  appearance  of 
great  rolls  arranged  in  parallel  lines,  and  pressed  close  against  one  another. 
In  their  centers  these  rolls  are  dark  in  color.     Blue  sky  may  be  seen  through 
the  intervening  spaces,  which  are  much  lighter  in  color.     (Roll-cumulus  in 
England,    Wulst-cumulus    in    Germany.)     Strato-cumulus    clouds    may    be 
distinguished  from  Nb  by  their  globular,  or  their  roll  appearance  and  by 
the  fact  that  they  are  not  generally  associated  with  rain. 

Strato-cumulus  clouds  usually  follow  a  winter  storm,  cover- 
ing the  sky  during  the  filling  of  a  low  barometer.  The  foregoing 
description  is  sufficiently  plain  and  clear  to  indicate  the  charac- 
ter and  appearance  of  strato-cumulus  clouds.  If  they  are  high 
enough,  however,  they  may  be  mistaken  for  alto-cumulus.  In 
such  a  case  it  might  be  correct  to  call  them  alto-cumuli.  Close 
to  the  horizon,  strato-cumulus  clouds  resemble  the  normal 
stratus  clouds  at  times,  but  they  are  much  higher.  Pretty 
nearly  every  transition  between  strato-cumulus  and  alto- 
cumulus clouds  may  be  observed. 

7.  NIMBUS  (Nb). — RAIN  CLOUDS. — A  thick  layer  of  dark  clouds,  without 
shape  and  with  ragged  edges,  from  which  steady  rain  or  snow  usually  falls. 

1  At  Blue  Hill  Observatory  thay  are  classed  as  alto-nimbus  when  rain  or 
snow  is  falling  from  them. 


80       THE    MOISTURE    OF    THE    AIR:     FOG    AND    CLOUD 

A  bank  of  cirro-stratus  clouds  in  the  west  is  apt  to  be  the  advance  of  a 
cyclonic  storm.  By  the  time  the  advancing  clouds  have  reached  the  eastern 
sky,  the  storm  is  close  at  hand.  Undulated  alto-stratus  clouds  form  under  the 


Ellerman,  photo. 

Thin,  undulated  alto-stratus  forming  above  a  fog,  or  stratus,  Mount 
Wilson,  Cal. 


cirrus  haze  and  these  very  shortly  develop  into  rain  clouds,  or  else  are  followed 
by  them.  A  winter  cyclonic  storm  may  be  likened  to  a  cone  with  its  apex 
tipped  one  hundred  miles  or  more  beyond  its  base. 


CLOUD  CLASSIFICATION    ;       />,  \     \  »!'/';£& 

Through  the  openings  in  these  clouds  an  upper  layer  of  Ci-St  or  A-St  may 
almost  invariably  be  seen.  If  a  layer  of  Nb  separates  into  shreds  in  a  strong 
wind,  or  if  small  loose  clouds  are  visible  floating  under  a  large  Nb,  the  cloud 
may  be  described  as  fracto-nimbus  (Fr-Nb),  the  "scud"  of  sailors. 

Inasmuch  as  the  sky  is  almost  always  wholly  overcast 
during  a  steady  rain  or  snow,  the  ragged  edges  are  rarely  visible. 
The  foregoing  description  is  hardly  true  of  tropical  rain  clouds 
with  their  sharp,  greasy-appearing  edges.  The  observer  will 
not  be  in  serious  error  in  designating  any  low  cloud  from  which 
rain  is  falling  as  nimbus.  The  flying  scud,  its  top  pointing  with 
the  wind,  drops  no  rain.  The  breaking  of  a  nimbus  usually 
denotes  the  clearing  of  a  storm ;  and  although  the  scud  is  rain- 
less, it  is  properly  nimbus  cloud  matter  though  not  "rain 
clouds." 

8.  CUMULUS  (Cu),  WOOL-PACK  CLOUDS. — Thick  clouds  of  which  the  upper 
surface  is  dome-shaped,  and  exhibits  protuberances  while  the  base  is  horizontal. 
These  clouds  appear  to  be  formed  by  a  diurnal  ascensional  movement  which 
is  almost  always  noticeable.     When  the  cloud  is  opposite  the  sun  the  surfaces 
facing  the  observer  have  a  greater  brilliance  than  the  margins  of  the  pro- 
tuberances.    When  the  light  falls  aslant,  as  is  usually  the  case,  these  clouds 
throw  deep  shadows;    when,  on  the  contrary,   the  clouds  are  on  the  same 
side  of  the  observer  as  the  sun,  they  appear  with  bright  edges. 

True  cumulus  has  well-defined  upper  and  lower  limits,  but  in  strong 
winds  a  broken  cloud  resembling  cumulus  is  often  seen,  in  which  the  detached 
portions  undergo  continual  change.  This  form  may  be  distinguished  by  the 
name  fracto-cumulus  (Fr-Cu). 

The  cumulus  cloud  with  its  flat  base  and  rounded  dome  is 
so  full  of  character  that  the  foregoing  description  is  ample.  It 
is  the  summer  cloud  of  the  temperate  zones  and  the  shower 
cloud  of  the  tropics.  To  the  unaided  eye  the  constant  motion 
of  the  cloud  matter  is  apparent;  with  a  field  glass  the  con- 
vectional  motion  is  plainly  visible  in  the  larger  clouds.  The 
cumulus  is  an  "ascensional"  cloud,  because  the  water  vapor 
is  carried  upward  until  cooling  brings  about  condensation. 
The  condensed  vapor  sinks  until  it  is  again  warmed  to  the 
temperature  of  vaporization. 

9.  CUMULO-NIMBUS   (Cu-Nb),  THUNDER-CLOUD,  SHOWER-CLOUD.— Heavy 
masses  of  cloud  rising  in  the  form  of  mountains,  turrets,  or  anvils,  generally 
surmounted  by  a  sheet  or  screen  of  fibrous  appearance  (false  cirrus}  and  having 
at  its  base  a  mass  of  cloud  similar  to  nimbus.     From  the  base  local  showers 
of  rain  or  snow  (occasionally  of  hail  or  soft  hail)  usually  fall.     Sometimes 
the  upper  edges  assume  the  compact  form  of  cumulus,  and  form  massive  peaks 


;      tHE    MOISTURE    OF    THE    AIR:    FOG    AND    CLOUD 


Cumulo-nimbus  cloud  resulting  from  eruption  of  Vesuvius.     Note  the 
heavy  rain  falling  from  lower  part  of  cloud. 


CLOUD  CLASSIFICATION    ;  ]  >'//   ,$%, 

round  which  delicate  "false  cirrus"  floats.  At  other  times  the  edges  them- 
selves separate  into  a  fringe  of  filaments  similar  to  cirrus  clouds.  This  last 
form  is  particularly  common  in  spring  showers. 

The  front  of  thunder-clouds  of  wide  extent  frequently  presents  the  form 
of  a  large  arc  spread  over  a  portion  of  a  uniformly  brighter  sky. 

The  difference  between  the  ordinary  cumulus  cloud  and  the 
cumulo-nimbus  is  mainly  one  of  depth  and  intensity  of  motion 
within  its  mass.  If  condensation  is  so  intense  that  its  water 
content  reaches  the  ground,  the  cloud  is  cumulo-nimbus.  But 
Humphreys  points  out  the  fact  that,  in  arid  regions,  where  the 
ground  is  very  warm,  a  well-developed  thunder-head  sinks  until 
the  excessive  warmth  vaporizes  and  scatters  it.  The  aborted 
cumulo-nimbus  has  been  observed  by  the  author.  On  the  other 
hand,  a  torrential  shower  may  fall  for  a  few  minutes  from  a 
tropical  cumulus  cloud — shallow  as  to  extent  and  without  the 
angry-appearing  cauliflower  head  of  the  ordinary  cumulo- 
nimbus. 

The  fibrous  mantle  that  hovers  over  the  top  of  the  cumulo- 
nimbus is  a  cloud  of  snow  flakes.  The  cloud  itself  is  usually, 
but  not  always,  a  thunder-storm.  The  observer  may  disregard 
all  theoretical  matters  and  record  it  as  a  cumulo-nimbus  if 
rain  is  falling  from  its  base.  The  marvelous  photograph  of  a 
thunder-storm  obtained  by  Lieutenant  W.  F.  Reed,  Jr.,  U.  S.  N. 
(p.  106),  surpasses  any  verbal  description  of  a  thunder- 
storm. 

The  strong  updraught  caused  by  forest  fires  and  burning 
strawstacks  has  resulted  in  the  formation  of  cumulus  clouds  that 
still  later  developed  into  typical  cumulo-nimbus  shower  clouds. 
The  eruption  of  Vesuvius  in  1872  created  a  series  of  cumulo- 
nimbus clouds  with  mammoth  cauliflower  heads.  Torrential 
rains  fell  on  the  leeward  side  of  the  cinder  cone  during  a  consider- 
able time. 

Experience  has  taught  the  airman  that  the  cumulo-nimbus 
cloud  is  an  object  to  be  avoided;  its  beautiful  exterior  hides  a 
generous  accumulation  of  holes  and  bumps.  The  turbulent 
cumulus  cloud  has  been  called  "the  most  treacherous  wild  beast 
of  the  air." 

10.  STRATUS  (St). — A  uniform  layer  of  cloud  resembling  a  fog,  but  not  resting 
on  the  ground.  When  this  sheet  is  broken  into  irregular  shreds  by  the  wind, 
or  by  the  summits  of  mountains,  it  may  be  distinguished  by  the  name  fracto- 
stratus  (Fr-St). 


$l'''."tHE:  MOISTURE   OF   THE   AIR:     FOG    AND    CLOUD 


CLOUD  CLASSIFICATION  85 

Tradition  has  made  the  long,  flat  cloud-streak  near  the 
horizon  the  type  form  of  stratus  cloud.  But  if  that  same  cloud- 
streak  were  overhead  it  would  appear  merely  as  a  low  cloud 
covering  more  or  less  of  the  sky.  When  a  fog  lifts,  it  forms  a 
stratus  cloud;  and  if  it  floats  away  toward  the  observer's 
horizon  it  becomes  a  long  gray  cloud  streak.  In  the  first  case 
one  is  looking  at  the  under  side  of  the  surface;  in  the  second,  at 
the  edge.  The  components  of  a  stratus  cloud  may  be  fog, 
smoke  or  dust — or  even  all  three. 

Qualifying  Descriptive  Terms. — Usage  in  the  matter  of 
descriptive  terms  is  not  uniform.  The  following  have  been 
suggested  :x 

Fibrous,  characteristic  of  streaks  of  falling  rain  or  snow  seen  from  a 
distance. 

Smooth,  characteristic  of  sheet-like  clouds. 

Flocculent,  scaly,  flaky,  in  small  tufts  (floccus,  a  tuft  of  wool). 

Waved,  or  in  rolls,  characteristic  of  waves  and  windrows  observable  in 
billow  clouds. 

Round-topped,  characteristic  of  the  summits  of  clouds  produced  by  rising 
currents. 

Down-bulged,  or  round-holed,  characteristic  of  the  lower  sides  of  clouds 
produced  by  down-draughts. 

Ragged,  characteristic  of  forming  and  of  evaporating  clouds  in  turbulent 
wind. 

Recording  Cloud  Conditions. — The  International  Cloud 
Committee  recommends  the  following  instructions  for  the 
guidance  of  observers: 

Kind  or  character. — Clouds  may  be  designated  by  name,  or  by  symbol, 
as  Ci-St,  for  cirro-stratus.  Where  doubt  exists,  the  number  of  the  picture 
in  the  classification  scheme  should  be  designated. 

Direction. — If  the  clouds  are  high  the  motion  may  be  observed  best  by 
noting  their  position  relative  to  a  fixed  object — a  tree,  or  a  flag-pole.  Where 
the  movement  of  the  cloud  is  very  slow,  a  rest  for  the  head  and  shoulders 
may  be  necessary.  The  direction  is  best  observed  when  clouds  are  near  the 
zenith.  The  movement  and  direction  of  horizon  clouds  are  apt  to  be  decep- 
tive, giving  to  the  observer  an  imperfect  perspective.  When  possible,  a 
nephoscope  should  be  used  if  the  direction  is  doubtful. 

1 C.  F.  Brooks,  Monthly  Weather  Review,  Sept.,  1920.  Seven  terms 
noted  above  are  used  for  form;  five — transparent,  semi-transparent,  medium, 
dense,  and  very  dense — describe  density;  three — coarse,  medium,  and  fine — 
indicate  the  degree  of  fineness.  These  terms,  while  they  do  not  alter  the 
International  Cloud  Committee's  classification,  add  very  materially  to  its 
clearness.  * 


86       THE    MOISTURE    OF    THE    AIR:     FOG    AND    CLOUD 


UNUSUAL  CLOUD  FORMS  87 

Radiant  Point  of  Upper  Clouds. — Though  apparently  in  radial  position, 
streamers  of  cirrus  clouds  are  actually  parallel.  The  radial  form  is  merely 
a  perspective.  The  apparent  point  of  convergence  should  be  noted  in  the 
same  manner  as  wind  direction;  as,  se,  or  nw. 

Undulatory  clouds. — If  the  clouds  show  parallel  and  equidistant  stria- 
tions,  such  as  suggest  a  succession  of  water  waves,  the  direction  of  the  striae 
should  be  noted;  and  if  more  than  one  system  of  striae  appear,  this  fact 
should  be  noted. 

Density  and  position  of  cirrus  forms. — The  cirro-stratus  haze  may  become 
a  dense  bank  of  .gray  in  its  lower  part.  It  is  desirable  that  its  density  be 
recorded  by  a  scale  of  intensity,  o  to  4;  and  also  that  the  cardinal  direction 
of  the  point  of  greatest  density  be  noted.  The  gathering  of  cirrus  clouds 
and  the  formation  of  a  cirro-stratus  bank  is  closely  connected  with  cyclonic 
storms. 

Unusual  Cloud  Forms. — Various  cloud  forms  that  are  not 
readily  classified  are  noted  by  every  observer: 

Billow  clouds,  or  windrow  clouds,  are  the  same  as  the  un- 
dulatory  clouds  noted  in  a  previous  paragraph.  The  name  is 
derived  from  their  wave-like  form.  Sometimes  they  are  at 
cirrus  height,  and  should  be  classed  as  cirrus  clouds.  For  the 
greater  part  they  form  at  lower  altitudes.  They  are  due  to 
cross-winds  in  plane  contact,  the  two  differing  in  temperature 
and  humidity. 

Crest  clouds  frequently  gather  about  the  summits  of  snow- 
clad  peaks.  They  are  frequently  observable  shrouding  the 
summits  of  Mounts  Hood  and  Rainier.  On  even  a  grander 
scale  they  envelop  the  summits  of  Popocatepetl  and  Ixtacci- 
huatl,  during  periods  of  still  air.  When  a  moist  wind  blows 
against  snow-clad  peaks,  a  stream  of  condensed  moisture  flows 
from  the  leeward  side,  forming  a  banner  cloud.  The  so-called 
"smoking"  of  Mounts  Hood  and  Rainier  is  a  cloud  banner  of 
this  sort. 

Mammillated,  or  mammato-cumulus  clouds,  are  globular 
projections  from  the  under  side  of  thunder-heads.  They  usually 
accompany  thunder-storms,  hailstorms  and  tornadoes.  A 
similar  waviness,  very  strong  in  character,  is  sometimes  observ- 
able in  the  bands  of  cirro-stratus  clouds  near  to  the  horizon. 

Scarf  clouds  are  the  feathery  wisps  that  sometimes  form  at 
the  summits  of  cumulus  clouds,  especially  those  of  the  storm 
type.  They  seem  to  increase  in  size  as  the  turbulence  within 
the  cumulus  cloud  increases,  and  sometimes  appear  like  a 
coverlet  over  its  top. 


88       THE    MOISTURE    OF    THE   AIR:    FOG    AND    CLOUD 


CLOUD  HEIGHTS  89 

Various  other  terms  such  as  lenticulate,  maculate,  flocculent, 
and  castellate,  are  used  by  observers.  Any  descriptive  term 
which  conveys  a  definite  meaning  is  permissible  in  recording 
cloud  observation.1 

Cloud  Heights. — Bigelow's  measurements  of  cloud  heights 
are  somewhat  greater  than  those  determined  by  the  Inter- 
national Cloud  Committee,  due  to  the  fact  that  the  measure- 
ments were  made  in  a  lower  latitude.  The  table  (p.  "),  taken 
from  Harm's  Lehrbuch  der  Meteorologie,  shows  that  the  altitudes 
of  the  various  cloud  levels  increase  from  polar  to  equatorial 
regions. 

The  level  of  each  type  of  cloud  is  a  level  of  maximum  cloud- 
iness; between  cloud  levels  are  levels  of  minimum  cloudiness. 
The  airman  may  find  that  neither  Dr.  Bigelow's  figures  nor 
those  of  the  International  Committee  apply  to  the  locality 
in  which  his  flights  are  made;  but  the  altitudes  of  maximum 
and  of  minimum  cloudiness  for  any  locality  are  not  far  from  the 
figures  noted  and  are  roughly  proportioned.  The  airman  will 
find  also  that  the  various  cloud  regions  are  thicker  as  one  ap- 
proaches equatorial  latitudes.  The  lowest  level  of  minimum 
cloudiness  is  that  between  "scud"  cloud  height  and  the  base 
height  of  stratus  clouds — from  300  feet  to  1200  feet. 

The  thunder-head  excepted,  the  lower  clouds  are  shallow; 
but  they  vary  greatly  in  depth.  A  mean  of  10,000  feet  (3000 
meters)  may  be  approximately  correct  for  their  depth,  but  it  is 
unsafe  as  an  estimated  depth  at  any  one  time.  The  fact  that 
the  highest  mountain  peaks  of  the  United  States  are  snow- 
capped shows  that  precipitation  occurs  at  an  altitude  of  about 
15,000  feet;  and  the  fact  that  observers  in  mountain  regions 
are  frequently  above  storm  clouds  is  evidence  that  the  cloud 
blanket  may  be  materially  less  than  10,000  feet  in  thickness. 

The  Distribution  of  Cloudiness. — In  the  Pacific  Coast 
region  of  the  United  States,  cloudiness  is  more  or  less  seasonal. 
Practically  all  the  lower  clouds  are  prevalent  during  the  winter 
months — that  is,  during  the  rainy  season.  During  the  summer 
the  lower  clouds  may  be  absent  for  weeks  at  a  time.  From  a 

4  .       " 

1  The  student  is  advised  to  become  familiar  with  A.  W.  Clayden's  article, 
"  Clouds,11  in  the  eleventh  edition  of  the  Encyclopedia  Britannica.  Clayden's 
modification  of  the  International  classification  is  merely  the  addition  of  de- 
scriptive terms. 


90       THE    MOISTURE    OF    THE    AIR:    FOG    AND    CLOUD 


CLOUD  FREQUENCY          i    ^  .-*     ,  >^^  91 

camp  in  the  Sierra  Nevada  Mountains  overlooking  the  Sacra- 
mento Valley,  clouds  formed  by  dust,  smoke  and  fog  are  some- 
times the  only  ones  visible  for  a  considerable  period. 

Along  the  Atlantic  Coast  and  the  Mississippi  Valley  a 
cloudless  sky  for  more  than  one  or  two  days  is  unusual;  for  a 
period  of  three  days  it  is  very  rare.  During  the  moist  periods 
of  midsummer  a  very  thin  cloud  veil  may  prevail  for  a  week 
or  more  at  a  time.  The  cloud  veil,  technically  a  "haze," 
is  too  thin  to  obscure  the  sun  visibly,  but  it  is  dense  enough 
to  prevent  radiation  to  a  considerable  extent.  During  the 
period  when  the  cloud  veil  prevails,  the  night  temperature  is 
from  5  to  10  degrees  higher  than  at  other  times,  and  the  amount 
of  insolation  is  almost  always  lowered. 

Arizona,  southern  California  and  southern  Nevada  consti- 
tute the  region  of  minimum  cloudiness  in  the  United  States. 
In  this  region  cloudless  skies  may  persist  for  a  month  or  more. 
The  observer  *who  watches  the  hygrometer  closely  will  acquire 
not  a  little  useful  information  on  clouds  and  their  relation  to 
atmospheric  moisture.  No  part  of  meteorology  is  more  fasci- 
nating than  the  study  of  clouds,  and  none  is  more  important  in 
forecasting  weather  changes. 


92.  "THE   MO'fSTURE   OF    THE   AIR:    FOG   AND    CLOUD 


•Is 


CHAPTER  IX 
THE  MOISTURE  OF  THE  AIR:    PRECIPITATION 

Dew   and    frost   are   commonly   regarded    as   condensation 
rain,  snow  and  hail  are  classed  as  precipitation.    So  much  of  the 
rain  and  snow  results  from  the  adiabatic  cooling  of  the  air — 
that  is,  cooling  by  an  updraught  of  warm  air — that  this  may  be 
considered  the  normal  cause  of  precipitation. 

A  mass  of  air  composing  an  updraught  is  cooled  at  the  rate 
of  I  degree  Fahrenheit  for  each  183  feet  of  ascent  (about  10.7 
degrees  centigrade  per  kilometer)  and  this  rate  does  not  vary 
much  in  the  first  1 0,000  feet.  When  the  rising  mass  has  reached 
the  level  where  it  is  at  the  temperature  of  saturation,  con- 
densation begins;  rain-clouds  form;  and,  from  the  coalescence 
of  cloud  matter,  rain-drops  or  snowflakes  fall.1 

The  most  remarkable  example  of  updraught,  adiabatic 
cooling,  condensation  and  precipitation  is  the  equatorial  cloud- 
belt.  It  is  the  updraught  of  the  Trade  Winds  and  consists  of  a 
zone  of  cumulus  clouds  several  hundred  miles  in  width.  In 
sailors'  vernacular,  it  is  the  belt  of  the  Doldrums.  Throughout 
most  of  its  width  rain  is  of  almost  daily  occurrence;  greasy- 
appearing  clouds,  with  sharp  edges,  hover  above  the  horizon 
about  noon,  and  steadily  mount  the  sky.  Wherever  they  are 

1  Rain-drops  vary  in  size  from  approximately  0.0004  inch  (o.oi  mm.)  to 
about  0.25  inch  (6.5  mm.)  in  diameter.  Drops  varying  in  size  from  very 
large  to  very  small  frequently  fall  in  the  course  of  one  shower.  Ordinarily 
the  drops  are  about  o.i  inch  (approximately  3  mm.)  in  diameter.  At  best, 
however,  these  dimensions  are  only  approximate.  Large  drops  are  shat- 
tered by  a  stiff  wind,  and  drops  a  quarter  of  an  inch  in  diameter  are  apt 
to  be  shattered  before  reaching  the  ground.  The  largest  drops  occur  in 
connection  with  thunder-storms.  A  shower  composed  of  fine  drops  much 
diffused  is  usually  termed  drizzle.  Very  fine  drops — droplets  that  are 
heavy  enough  to  fall — are  properly  called  mist.  These  droplets  are  larger 
than  those  of  fog,  the  latter  being  floating  and  not  falling  matter.  A  thin 
fog  is  also  called  mist,  in  Weather  Bureau  nomenclature. 

93 


94 


THE  MOISTURE  OF  THE  AIR:    PRECIPITATION 


SEASONAL  RAINFALL  '  '9'£ 

in  an  overhead  position  rain  is  falling  in  heavy  showers.  The 
passage  of  the  cloud-belt  north  and  south  provides  an  unusual 
and  an  interesting  distribution  of  rainfall.  Roughly  speaking, 
the  cloud-belt  halts  and  turns  backward  in  the  latitude  of  each 
tropic;  in  these  latitudes,  therefore,  there  is  theoretically  one 
rainy  season  each  year.  Between  the  tropical  circles  the  cloud- 
belt  passes  twice,  resulting  in  two  rainy  and  two  dry  seasons — 
in  some  localities  strongly  marked,  in  others  not  so  distinguish- 
able. 

South  of  the  equator  the  zone  of  constant  rajjns  is  not  so 
well  marked  as  it  is  north  of  the  equator;  moreover,  the  belt 
of  easterly  winds  at  times  covers  the  whole  of  the  Gulf  of 
Mexico.  In  general,  the  lands  of  the  Torrid  Zone  receive  an 
average  of  about  100  inches  of  rain  per  year — rather  more  in 
the  northern  than  in  the  southern  part. 

In  the  temperate  zones,  on  the  coasts  facing  westerly  sea 
winds,  the  rainfall  for  the  greater  part  is  seasonal.  Along  the 
Pacific  Coast  of  the  United  States,  it  increases  with  the  lati- 
tude. Thus  at  San  Diego,  California,  the  annual  fall  is  10 
inches;  at  Los  Angeles,  16  inches;  at  San  Francisco,  23  inches; 
at  Portland,  Oregon,  45  inches;  at  the  coast  stations  of  Alaska 
from  80  inches  to  more  than  100  inches.  At  San  Diego  prac- 
tically all  the  rain  falls  between  October  15  and  April  15;  in 
the  seventy- two  years  from  1850  to  1912,  a  shower  amounting 
to  more  than  a  trace  of  rain  fell  only  twenty  times  in  July.  In 
San  Francisco  there  is  an  average  of  about  one  rainy  day  in 
July;  in  Portland,  Oregon,  the  July  rainfall  averages  0.54 
inch;  in  Seattle  it  is  0.69  inch. 

On  the  west  coast  of  Europe,  owing  chiefly  to  higher  lati- 
tude, the  seasonal  character  of  the  annual  rainfall  is  not  so 
marked  as  in  California.  In  Portugal  and  southern  Spain, 
most  of  the  rain  falls  in  the  winter  months.  At  Bordeaux,  a 
little  higher  in  latitude  than  Seattle,  the  July  rainfall  is  in  excess 
of  2  inches  every  month  in  the  year.  But  while  the  average  of 
the  winter  months  in  Seattle  is  above  5  inches,  in  Bordeaux 
it  is  about  3  inches. 

The  thirtieth  parallel  crosses  the  northern  part  of  Mexico 
and  also  the  northern  part  of  Africa.  A  zone  several  hundred 
miles  in  width  along  this  parallel — in  sailors'  vernacular,  the 
belt  of  ' 'horse  latitudes" — is  the  region  of  high  barometric 


THE  "MOiSTCRE  OF  THE  AIR:    PRECIPITATION 


THE  PRECIPITATION  OF  CYCLONIC  STORMS      ''      §7 

pressure,  and  the  belt  of  descending  air.  The  region  covered 
by  it  is  one  of  calms  over  the  sea  and  of  light,  variable  winds 
over  the  land.  Along  the  Pacific  Coast  from  San  Diego  almost 
to  Manzanillo,  Mexico,  the  yearly  rainfall  is  very  light  and 
uncertain.  Along  the  Atlantic  Coast  of  Africa,  the  region  of 
calms  is  a  desert.  Eastern  Mexico  receives  a  more  generous 
rainfall;  and  southern  Mexico  and  the  Central  American 
states  are  within  the  zone  of  rain-bearing  winds. 

Southern  coasts  in  general  have  an  abundance  of  rain. 
Along  the  Indian  Ocean  and  the  Guinea  coast  the  rainfall  is 
seasonal.  On  the  Gulf  Coast  of  the  United  States  rain  falls 
pretty  evenly  throughout  the  year. 

The  Precipitation  of  Cyclonic  Storms. — Cyclonic  storms, 
or  lows,  are  rather  more  frequent  in  the  United  States  and 
Canada  than  in  Europe.  They  are  much  more  frequent  in  oc- 
currence east  of  the  Western  Highlands,  and  they  also  are  much 
more  energetic. 

Rather  more  than  half  of  the  storm-whirls  of  this  sort  are 
noted  first  between  the  Columbia  River  and  the  Strait  of  Juan 
de  Fuca — not  because  they  do  not  occur  elsewhere,  but  for  the 
reason  that  there  are  fewer  weather  stations  north  of  Van- 
couver. When  a  low  is  crossing  the  mountains  it  is  in  a  region 
of  dry  air;  therefore  it  does  not  possess  much  energy.  When 
it  has  crossed  the  Rocky  Mountains  it  is  in  a  region  where  both 
the  absolute  and  the  relative  humidity  are  greater.  Therefore 
it  is  apt  to  develop  a  much  greater  energy;  for  the  latent  heat 
set  free  by  the  condensation  of  water  vapor  is  the  fuel  of  a 
cyclone.  The  northerly  cyclone  usually  traverses  the  Great 
Lakes,  where  the  increased  humidity  imparts  greater  precipita- 
tion, finally  moving  out  into  the  Atlantic.  As  a  rule,  the  rain- 
fall of  such  storms  is  not  very  heavy.  It  may  drop  a  little  more 
than  I  inch  of  rain  over  the  track  of  its  passage,  but  usually  the 
precipitation  is  not  much  greater. 

The  more  southerly  cyclones  frequently  bend  towards  the 
Gulf  of  Mexico,  and  begin  to  curve  towards  the  northeast 
after  passing  the  Mississippi  River.  They  are  much  more 
energetic  than  the  northerly  storms  and  drop  perhaps  as  much 
as  2  inches  of  rain  along  their  tracks.  In  various  instances  a 
storm  first  discovered  in  the  plains  of  Texas  finally  travels  a 
course  between  the  St.  Lawrence  River  and  the  coast.  In- 


'MOlSTfrRE  OF  THE  AIR:    PRECIPITATION 


SNOW  '09J 

asmuch  as  this  track  covers  a  region  of  great  moisture,  the 
rainfall  is  apt  to  be  very  heavy — sometimes  more  than 
3  inches. 

The  severest  cyclonic  storms  are  the  West  Indian  hurricanes 
and  the  typhoons  of  the  China  coast.  Throughout  their 
courses  they  move  through  regions  of  very  moist  air.  In  these 
storms,  the  velocity  of  the  wind  results  from  a  very  rapid  up- 
draught.  The  precipitation,  therefore,  is  excessive.  In  the 
vicinity  of  the  Gulf  Coast  of  the  United  States,  from  8  to  10 


Bentley  photo. 
Snow  crystals,  magnified  about  50  diameters,  Jericho,  Vt. 

inches  of  rain  may  fall  during  the  passage  of  a  hurricane  storm, 
and  a  downpour  of  4  or  5  inches  is  usual. 

In  the  United  States,  cyclonic  storms  are  more  characteristic 
of  winter  than  of  summer  weather;  they  are  therefore  usually 
described  as  winter  storms.  The  precipitation  may  consist  of 
rain,  sleet  or  snow — rarely,  if  ever,  of  hail. 

Snow. — When  condensation  below  32°  F  (o°  C)  occurs,  the 
precipitation  takes  the  form  of  the  ice  crystals  popularly  known 
as  snowflakes.  They  form  in  almost  infinite  variety,  but  they 
may  usually  be  classified  as  tabular  (disk-shaped)  or  colum- 


:T'HE 


OF  THE  AIR:    PRECIPITATION 


SNOW 


nar.1  Normal  crystals  are  six-sided  or  six-pointed ;  the  angles  are 
usually  60°  or  120°.  The  snowflakes  of  ordinary  storms  consist 
of  tangled  masses  of  broken  crystals.  They  are  at  their  best 
when  the  temperature  is  not  higher  than  25°  F  and  the  air  is 
still.  The  flakes  should  be  caught  on  black  cloth.  If  a  micro- 
scope is  used,  it  must  be  used  in  a  place  where  the  temperature 
is  below  the  freezing  point.  If  photo-micrographs  are  made,  a 
low  power  objective — 2-inch  or  4-inch — gives  excellent  results. 

Occasionally  the  snowflakes  take  the  forms  of  soft  pellets — 
the  graupeln  of  the  German  meteorologist.  At  other  times  they 
are  half-melted,  but  retain  traces  of  crystallization.  The 
presence  of  slowly  falling  snow  crystals  during  fairly  clear 
weather  is  common  in  many  localities.  It  is  the  greatly- 
dreaded  poguenib  of  the  far-western  Indian,  who  associates  it 
with  pneumonia. 

Snowfalls  have  been  recorded  in  every  state  in  the  Union. 
They  occur  occasionally  along  the  Gulf  Coast  between  Pensa- 
cola  and  Brownsville.  Snow  has  fallen  in  Florida  as  far  south 
as  Fort  Myers.2  A  line  drawn  from  Savannah  through  San 
Antonio,  El  Paso,  and  Yuma  to  San  Francisco  marks  roughly 
the  limit  south  of  which  snow  seldom  falls.  South  of  the 
thirty-fifth  parallel,  snow  rarely  lies  on  the  ground  more  than 
a  day  or  two.  At  New  Orleans  a  measureable  snowfall  occurs 
about  once  in  fifteen  years.  It  is  about  as  frequent  in  the  city 
of  Los  Angeles,  although  the  mountain  summits  in  the  vicinity 
occasionally  are  snow-clad. 

In  the  vicinity  of  the  Great  Lakes  the  ground  is  covered 
most  of  the  winter.  In  the  New  England  and  Middle  Atlantic 
states  the  annual  snowfall  is  7  to  8  feet.  It  decreases  toward 
the  west,  being  about  2  feet  in  North  Dakota.  In  the  basin 
region  of  the  Rocky  Mountain  States  snowfalls  occur  at  long 
intervals  only;  in  the  plateau  region  they  may  be  expected 
yearly  on  the  range  summits.  The  heaviest  snowfalls  occur 
along  the  northern  Rocky  Mountain  and  the  Sierra  Nevada 

1  A  remarkable  collection  of  photographs  of  snow-flakes  has  been  made 
by  W.  A.  Bentley,  and  another  by  J.  C.  Shedd.     The  latter  is  published  in 
the    Monthly    Weather    Review,    October,    1919.     Professor    Shedd    classifies 
snow-flakes  as  first-,  second-,  and  third-growth  crystals.     They  have  been 
classified  also  as  columnar,  doublets,  and  pyramidal  crystals. 

2  On  March  6,  1843,  fifteen  inches  of  snow  fell  at  Augusta,  Georgia. 


F  THE  AIR:    PRECIPITATION 

Range  summits;  from  10  to  30  feet  may  be  estimated  as  the 
annual  fall.  The  amount  varies  greatly  from  year  to  year; 
at  Summit,  California,  60  feet  of  snow  fell  during  the  winter 
of  1879-80. 

On  the  Pacific  Coast  slope  the  yearly  snowfall  in  the  moun- 
tains is  a  matter  of  great  importance.  Since  the  construction 
of  the  various  irrigation  projects  in  the  arid  region,  humanity 
is  realizing  more  and  more  the  dependence  of  productive  lands, 
not  only  on  the  yearly  amount  of  snow-fall,  but  on  the  con- 
servation of  the  melting  snow,  as  well.  In  the  arid  regions  of 
the  United  States,  the  winter  snowfall  is  the  moisture  of  the 
summer  crops. 

Except  at  great  altitudes,  practically  all  the  snow  falls 
between  the  first  of  December  and  the  middle  of  April  in  the 
zone  of  latitude  that  includes  the  New  England  States  and  New 
York.1  Flurries  of  snow  occur  in  May  as  far  west  as  the  Rocky 
Mountains;  and  at  elevations  of  2000  feet  or  more  they  occur 
in  June. 

Sleet;  Ice  Storms;  White  Storms. — In  Weather  Bureau 
nomenclature,  sleet  consists  of  small  pellets  of  ice,  apparently 
formed  when  rain-drops  are  frozen  in  passing  through  a  stratum 
of  cold  air  next  the  ground.  Usually  the  pellets  are  not  larger 
than  duck  shot;  occasionally  they  are  the  size  of  peas.  Sleet 
has  been  reported  as  hail  so  frequently  that  the  Weather 
Bureau  has  issued  an  explanatory  pamphlet  calling  attention  to 
the  fact  that  the  ice  pellets  of  sleet  differ  materially  in  structure 
from  hailstones.  Ice  pellets  may  contain  enough  air  to  give 
them  a  whitish  opaque  appearance;  therefore  they  are  likely 
to  deceive  observers.  Sleet  storms  are  very  apt  to  occur  in 
the  morning,  when  the  temperature  is  at  its  daily  minimum, 
but  this  is  by  no  means  always  the  case.  Sleet  may  occur 
when  a  cold  wave  flows  under  warm,  moist  air;  it  is  likely  to 
result  when  a  warm  southerly  wind  flows  over  the  top  of  very 
cold  surface  air. 

Sleet  is  often  mixed  with  rain;  at  such  times  it  forms  an  ice- 
coating  on  the  ground,  making  a  surface  that  is  more  or  less 
pebbly.  Frequently  it  happens  that  the  rain-drops  are  not 

1  On  the  8th  of  June,  1816,  snow  fell  in  all  parts  of  Vermont;  on  the 
uplands  it  was  5  or  6  inches  deep.  It  was  accompanied  by  a  hard  frost. — 
Thompson's  History  of  Vermont. 


HAIL  103 

frozen  in  their  fall,  but  congeal  as  they  strike.  In  this  way, 
ground,  sidewalks,  trees  and  other  surfaces  become  covered 
with  a  coating  of  ice.  Weather  Bureau  practise  and  popular 
consent  join  in  designating  this  form  of  precipitation  as  an 
ice  storm.  The  ice  storm  is  apt  to  be  followed  by  the  destruc- 
tion of  tree  branches  snapped  off  by  the  wind,  and  by  an  unusual 
number  of  accidents  in  city  street  traffic. 

Several  conditions  of  temperature  and  precipitation  may 
result  in  an  ice  storm.  If  the  rain-drops  fall  through  a  stratum 
of  air  below  freezing  temperature  and  strike  an  object  whose 
surface  is  also  below  freezing  temperature  a  varnish  of  ice  will 
be  formed,  and  it  is  likely  to  increase  in  thickness  so  long  as 
precipitation  continues.  When  the  temperature  is  very  slightly 
above  the  freezing  point,  and  the  surface  air  is  dry,  it  is  possible 
that  rapid  evaporation  may  chill  the  varnish  of  water  below 
the  freezing  point  and  change  it  to  ice.  Rain-drops  in  the  air 
may  be  chilled  to  a  temperature  several  degrees  below  the 
freezing  point;  they  change  to  ice  instaritly  as  they  strike.1 

Damp  snow  and  snow  falling  on  tree  limbs,  poles  and  wires 
whose  temperature  is  slightly  above  freezing,  is  very  apt  to  cling 
to  them.  The  weight  of  the  accumulated  snow  may  be  suffi- 
cient to  break  tree  limbs  and  line  wires.  Not  only  is  there  a 
considerable  material  damage;  there  is  also  a  troublesome  and 
expensive  interruption  of  communication.  Popularly,  the  con- 
dition is  known  as  a  white  storm. 

A  temperature  materially  below  32°  F  following  a  white 
storm  is  apt  to  result  in  much  damage  to  shade  trees  and  or- 
chards. Branches  will  bend  freely,  as  a  rule,  when  the  tem- 
perature is  above  freezing;  but  they  become  brittle  under  intense 
cold  if  coated  with  ice.  The  distinction  between  an  ice  storm 
and  a  white  storm  is  chiefly  one  of  appearance.  The  Weather 
Bureau  makes  no  distinction  between  them. 

Hail. — Hail  is  a  product  of  thunder-storms.  The  hailstone 
consists  of  alternate  concentric  layers  of  snow  and  ice.  The 
manner  of  the  formation  of  the  hailstone  is  conjectural.  About 
the  only  thing  of  which  one  may  be  certain  is  that  the  hailstone 
is  alternately  in  layers  of  moist  air  below  the  freezing  point  and 

1  The  Blue  Hill  Observatory  reports  rain  falling  when  the  temperature 
of  the  air  was  about  15°  F — a  very  unusual  phenomenon.  It  is  not  likely, 
however,  that  the  rain-drops  had  reached  a  temperature  much  below  freezing. 


104 


THE  MOISTURE  OF  THE  AIR:    PRECIPITATION 


layers  of  warmer  air — that  is,  it  is  whirled  through  alternate 
layers  of  snowy  air  and  of  misty  air.  The  updraught  that 
occurs  during  thunder-storms  shows  that  such  a  movement  takes 
place  in  cumulo-nimbus  clouds;  and  when  the  hailstones  become 
too  heavy  to  be  carried  by  the  updraught  they  fall  to  the 
ground. 

Hailstones  usually  vary  in  size  from  a  quarter  of  an  inch 

to  half  an  inch  in  diameter. 
They  are  very  rarely  as  much  as 
an  inch  in  diameter.  In  a  few 
instances  single  stones  more  than 
two  inches  in  diameter  have 
been  reported.  In  many  in- 
stances several  hailstones  are 
frozen  together,  and  hailstones 
"as  large  as  a  hen's  egg"  are 
formed  in  this  manner.  Hail- 
storms are  rarely  more  than  a 
few  minutes  in  duration. 

They  occur  usually  in  the 
southeast  quadrant  of  a  cyclonic 
storm,  having  the  same  relation 
to  the  area  of  low  barometer  as  does  the  tornado.  The  path 
of  the  hailstorm  is  rarely  more  than  3  or  4  miles  wide — some- 
times not  more  than  half  a  mile — and  it  may  traverse  a  distance 
of  25  or  30  miles,  or  more. 

Sometimes  the  hail  is  scattered  in  windrows;  and  many 
cases  in  the  United  States  have  been  reported  where  the  wind- 
rows were  several  rods  in  width  and  more  than  2  feet  deep. 
Near  St.  Quentin,  France,  a  windrow  more  than  a  mile  long 
left  a  mass  of  ice  which  did  not  disappear  for  several  days. 

In  western  Europe  hailstorms  are  very  destructive  to  vine- 
yards and  growing  crops.  For  many  years  the  practise  of 
"bombarding  the  air"  was  followed.  Long-barreled  mortars 
with  bell-shaped  bores  were  charged  heavily  with  powder  and 
aimed  vertically.  At  times  when  storms  were  expected,  thou- 
sands of  charges  were  fired  into  the  air  with  the  expectation 
that  the  resulting  convection  of  the  air  might  prevent  the  for- 
mation of  hail.  There  is  no  evidence  to  show  that  the  practise 
prevents  hailstorms. 


After  Red-way. 
Hailstone;  sectional  view. 


CLOUDBURSTS  105 

In  the  present  state  of  human  knowledge,  forecasts  of  hail- 
storms cannot  be  made.  The  Weather  Bureau  i;j  making 
efforts  to  gain  all  possible  information  concerning  date,  time, 
duration,  extent  of  area  and  path  of  hailstorms.  It  is  pretty 
well  established,  however,  that  hailstorms  are  more  frequent 
in  certain  regions  than  in  others;  and  that  in  certain  limited 
areas  in  these  regions  of  greatest  frequency  they  are  more 
destructive  than  in  other  areas. 

Cloudbursts. — The  cloudburst  is  an  excessive  downpour  of 
rain,  in  which  the  water  seems  to  fall  in  masses  rather  than 
in  drops.  Cloudbursts  are  rare;  the  area  covered  is  small; 
the  duration  is  a  matter  of  a  few  moments  only.1  Only  in  a 
few  cases  have  trustworthy  measurements  of  the  amount  of 
precipitation  been  made.  The  ordinary  barrel  gauge  would 
probably  give  a  result  at  least  80  per  cent  true.  The  majority 
of  recording  gauges  are  of  but  little  use  in  such  storms.  More- 
over, the  cloudburst  does  not  always  select  for  its  performance 
a  locality  where  Weather  Bureau  stations  are  in  evidence. 

The  origin  of  the  cloudburst  is  not  certainly  known.  To 
call  it  an  exaggerated  thunder-storm  may  express  a  truth  in 
some  cases;  certainly  not  in  every  case.  All  the  water  in  an 
overhead  saturated  air  at  a  temperature  of  70°  F  over  the  area 
covered  by  the  downpour  would  not  make  a  rainfall  sufficient 
to  account  for  the  water  dropped  by  a  cloudburst. 

1A  mining  engineer  in  Arizona  relates  the  following:  "The  day,  up  to 
3  o'clock,  had  been  moist — to  the  extent  that  distant  objects  possessed 
atmosphere — that  is,  there  was  not  the  illusion  of  nearness  which  a  very 
dry  air  gives.  In  mid-afternoon  there  came  a  sudden  darkening  of  the 
sky,  a  light  patter  of  rain,  and  then  a  downpour  so  torrential  that  further 
progress  along  the  trail  was  out  of  question.  The  loose  rock  waste  seemed 
to  be  washed  out  from  under  the  horse's  hoofs,  and  a  boss  of  rock  near  by 
seemed  to  be  the  only  safe  place.  It  was  impossible  to  see  anything  more 
than  a  few  rods  ahead.  The  downpour  lasted  for  not  more  than  fifteen 
minutes.  To  say  that  the  amount  was  6  inches  is  merely  a  guess.  Much 
of  the  trail  was  washed  away  and  badly  gullied.  Final  Creek  was  a  roaring 
torrent;  to  have  attempted  crossing  it  would  have  been  instant  death. 
Across  the  summit,  on  the  other  side  of  the  range,  not  a  drop  of  rain  fell." 

Another  traveler  wrote:  "A  heavy  cloud  had  been  hovering  over  Pilot 
Range  for  several  hours,  and  we  were  not  surprised  to  hear  a  low  moan  which 
soon  became  a  roar.  So  we  climbed  out  of  the  arroya  in  quick  time.  In 
a  very  few  moments,  there  came  a  torrent  that  would  have  carried  a  ton 
boulder  down  the  course.  The  cloud  over  Pilot  Knob  had  dropped  its  shower 
and  the  sink  below  was  full  of  water — the  first  time,  perhaps,  in  fifty  years. ' ' 


106 


THE  MOISTURE  OF  THE  AIR:    PRECIPITATION} 


SUMMER  PRECIPITATION  107 

It  has  been  pointed  out  that  the  cloudburst  may  be  derived 
from  the  contents  of  a  waterspout  carried  inland  for  a  long 
way  and  dumped  upon  the  nearest  mountain  crest  which  has 
a  temperature  low  enough  to  chill  it.  This  may  be  an  ex- 
planation, but  one  is  not  certain  that  it  is  the  real  one.  Any 
explanation  must  take  into  account  the  fact  that  an  ordinary 
rain  cloud  cannot  hold  the  moisture  that  is  precipitated  in  a 
cloudburst. 

Summer  Precipitation. — The  rainfall  of  summer  months 
within  the  United  States  is  rarely  a  result  of  cyclonic  move- 
ments of  the  air.  For  the  greater  part,  it  is  due  to  the  up- 
draughts  that  result  from  surface  heating;  and  this  also  is  the 
cause  of  most  of  the  tropical  rains.  Summer  showers  are  apt 
to  be  sporadic  in  character,  and  the  area  covered  may  be  small. 
It  is  not  uncommon  to  find  a  rainfall  of  2  or  3  inches  at  one  lo- 
cality while  scarcely  more  than  a  trace  falls  at  another  locality 
only  a  few  miles  distant.1  Occasionally  the  daily  weather 
map  shows  half  a  dozen  areas  scattered  over  the  eastern  half 
of  the  United  States  in  which  rain  is  falling;  less  frequently,  a 
belt  200  miles  or  more  in  width  extending  from  the  Gulf  Coast 
to  the  Canadian  border,  sweeps  eastward  from  the  Mississippi 
Valley. 

In  summer  the  updraught,  though  strong,  is  more  or  less 
local,  occurring  over  comparatively  small  areas.  In  winter  the 
updraught,  though  weak,  may  involve  an  area  more  than  500 
miles  in  diameter. 

1  In  the  past  few  years  insurance  against  rain  has  become  very  common 
during  summer  months.  Clubs  and  outing  associations  thus  protect  them- 
selves against  the  losses  of  revenue  which  a  rainfall  might  cause.  In  many 
instances  the  serious  error  of  determining  the  rainfall  by  the  record  of  a 
rain-gauge  a  dozen  miles  distant  has  led  to  expensive  litigation.  In  various 
instances,  heavy  showers  have  occurred  at  the  locality  covered  by  the  policy, 
while  merely  a  trace  fell  at  the  station  where  the  precipitation  was  recorded. 
Granted  that  insurance  against  rain  is  a  legitimate  business,  it  is  evident 
that  the  installation  of  a  gauge  at  the  locality  covered  is  the  only  way  by 
which  the  amount  of  rain  can  be  determined. 


CHAPTER  X 
ATMOSPHERIC    ELECTRICITY:    OPTICAL   PHENOMENA 

ATMOSPHERIC  ELECTRICITY 

Under  ordinary  conditions  the  electricity  of  the  air  is  positive 
in  relation  to  the  ground  and  the  oceans.  Its  potential  does  not 
vary  greatly,  being  rather  higher  in  winter  than  in  summer — 
a  change  which  might  be  considered  normal.  During  rainfall 
or  snowfall  the  potential  usually  is  unsteady,  varying  rapidly 
between  positive  and  negative.  The  changes  are  quiet;  in 
ordinary  cases  they  can  be  detected  only  by  means  of  sensitive 
electrometers  designed  for  the  purpose. 

Ether  Waves. — From  time  to  time  there  are  sharp  but  slight 
.variations  in  the  electric  potential  both  of  the  earth  and  of  the 
air.  The  former  are  created  by  the  "earth  currents"  which,  in 
the  time  when  the  telegraph  was  operated  by  grounded  battery 
circuits,  were  the  bane  of  the  telegrapher.  The  sharp  variations 
of  the  atmospheric  potential  are  known  as  "static  waves,"  or 
"ether  waves";  they  are  the  most  common  obstacle  in  radio- 
telegraphy  and  telephony. 

The  ether  waves  of  atmospheric  electricity  apparently  have 
little  or  no  effect  upon  the  activities  of  life;  they  also  seem  to 
be  unimportant  to  meteorology,  except  as  their  increasing 
frequency  may  possibly  indicate  the  approach  of  a  thunder- 
storm.1 Ether  waves  of  the  Hertzian  type,  caught  at  a  distance 

1  Ether  waves  are  made  audible  by  means  of  the  mineral  detectors  for- 
merly used  by  radio-telegraphers,  and  by  the  use  of  the  various  devices 
known  as  audions.  The  passage  of  an  ether  wave  caught  by  the  antennae 
gives  a  distinctive  hissing  sound  in  the  telephone.  A  strong  wave  illu- 
minates a  Geissler  tube  placed  in  the  circuit.  A  more  striking  result  may 
be  obtained  by  using  a  coherer  and  a  relay  with  one  or  two  dry  cells.  The 
passage  of  the  ether  wave  from  the  antennae  to  the  ground  electrifies  the 
filings  in  the  coherer  to  the  extent  that  a  battery  circuit  is  formed,  which 
closes  the  relay.  The  closing  of  the  relay  may  be  used  to  close  a  secondary 
bell  circuit,  or  to  communicate  any  other  desired  signal.  A  drop  of  clean 

108 


ELECTRICAL  CONDITIONS  OF  THE  AIR 


109 


of  50  miles,  more  or  less,  from  the  thunder-storm,  may  be  com- 
pared to  the  ground  swell  of  the  sea  formed  by  a  distant  storm. 

Electrical  Conditions  of  the  Air. — Humidity  may  or  may  not 
affect  the  potential  of  the 
air  materially,  but  it  affects 
the  conductivity  greatly. 
Dry  air  is  a  poor  conductor; 
and  dust  particles,  unable  to 
discharge  their  load  of  elec- 
tricity, are  strongly  repel- 
lant  and  remain  suspended 
in  the  air,  sometimes  for 
several  days.  The  desert 
simoon  is  followed  by  a  con- 
dition which  keeps  the  air  at 
a  high  potential,  with  a 
highly  electrified  dust,  for 
several  days. 

At  the  trading-posts  in 
the  Colorado  and  Mohave 
Deserts,  after  a  simoon  has 
passed,  metal  containers  on 
wooden  shelves  become  con- 
densers of  a  considerable 
capacity.  Horses'  manes 
and  tails  stare  like  fright 
wigs,  and  sparks  crackle  to 
any  ground  conductor  that 
may  be  touched.  At  such 
times,  strong  earth  currents 
may  be  detected,  and  their 


Ether  wave  indicator,  Meteorological 
Laboratory.  The  wire  at  the  upper 
right  of  the  spark  gap  leads  to  the 
aerial.  The  lower  binding  post  of  the 
condenser  leads  to  the  ground.  The 
coherer  is  shown  above  the  bell 
hammer. 


influence  may  be  felt  many 
miles  distant. 

In  a  case  of  this  sort  the 
high  potential  is  local — that 
is,  it  is  confined  to  the  mass  of  dry  desert  air,  and  this  mass  of  air 


mercury  between  two  iron  plugs  within  a  glass  tube  makes  an  excellent 
coherer,  when  placed  in  a  circuit.  Lighting  companies  sometimes  make  use 
of  such  "storm  indicators"  to  guide  them  in  generating  the  additional  cur- 
rent made  necessary  by  the  darkness  accompanying  summer  storms. 


110     ATMOSPHERIC    ELECTRICITY:    OPTICAL   PHENOMENA 

is  practically  a  great  condenser  which  has  been  charged  to  a 
potential  much  higher  than  that  of  the  air  surrounding.  In 
time — from  twelve  to  forty-eight  hours — the  high  electric  charge 
disappears,  and  the  potential  sinks  to  normal.  The  question — 
"How  can  the  air,  which  is  composed  mainly  of  gases,  become 
a  condenser  and  hold  a  charge  of  electricity?" — is  not  difficult 
to  answer.  The  static  charge  of  an  electrified  body  practically 
is  on  the  surface  of  the  body.  Every  substance  must  possess  sur- 
face and  the  molecules  of  the  gases  composing  the  air  are  not  an 
exception;  neither  are  the  dust  particles  floating  in  the  air;  there- 
fore they  act  as  condensers,  receiving  and  discharging  electrons. 

Just  as  water,  by  seeking  its  own  level,  acquires  an  even 
and  uniform  pressure,  so  the  electricity  of  the  air  seeks  an  even 
and  uniform  potential.  If  a  body  of  cold,  dry  and  highly 
charged  air  flows  into  a  region  of  low  potential,  or  into  one 
oppositely  charged,  an  interchange,  or  flow  of  electricity, 
results.  The  interchange  may  be  so  quiet  that  it  escapes  notice;1 
on  the  other  hand,  it  may  be  violent  enough  to  produce  strong 
electrical  discharges. 

The  origin  and  source  of  atmospheric  electricity  is  still  a 
problem  to  be  solved;  so  also  is  the  origin  of  earth  electricity. 
To  the  best  of  human  knowledge,  the  earth  is  constantly  giving 
off  negative  electricity,  and  receiving  none  in  return,  except 
that  which  is  brought  down  by  rain,  or  by  snow,  or  by  light- 
ning strokes  which  pass  from  the  clouds  to  the  earth.  The 
reason  therefor  is  not  known. 

It  has  been  found  that  a  rainstorm  carries  to  the  earth 
about  3.5  times  as  much  positive  as  negative  electricity;2  and 
that  positively  charged  snow  falls  more  frequently  than  that 
which  is  negatively  charged.  A  reason  therefor  certainly  ex- 
ists, but  it  is  not  known.  The  breaking  of  large  drops  of  water 
into  spray  is  accompanied  by  the  production  both  of  positive 
and  negative  electricity.  Conversely,  when  fine  spray  is  charged 
with  electricity,  the  spray  immediately  coalesces  into  very  large 
drops  of  water. 

Extra-terrestrial  Influences  in  Atmospheric  Electricity.— 
The  fact  that  rapid  movements  in  sun  spots  and  similar  dis- 

1  The  interchange,  no  matter  how  quiet,  will  operate  the  apparatus  de- 
scribed on  p.  108. 

2  The  records  of  Dr.  C.  G.  Simpson,  London  Meteorological  Office. 


EXTRA-TERRESTRIAL  INFLUENCES  111 

turbances  in  the  photosphere,  or  envelope  of  the  sun,  are 
coincident  with  magnetic  storms  and  earth  currents  leads  to 
the  belief  that  solar  influences  at  times  are  factors  in  at- 
mospheric electricity.  It  is  not  safe  to  infer,  that  because  of 
this  fact,  the  electricity  of  the  earth  and  its  atmosphere  are 
derived  from  the  sun.  Practically  all  evidence  is  contrary  to 
such  an  assumption;  nevertheless,  there  seems  no  reason  to 
doubt  that  high-frequency  waves  generated  in  the  sun  reach 
the  earth. 

The  phenomenon  known  as  the  aurora  borealis  (aurora 
polaris),  more  commonly  called  "northern  lights,"  is  most 
frequently  observed  during  great  disturbances  in  the  sun's 
photosphere.  But  it  is  by  no  means  certain  that  the  display, 
which  is  electrical,  is  due  to  solar  causes.  The  belief  that  the 
aurora  is  of  solar  cause,  however,  is  held  by  many  physicists. 

The  height  of  the  aurora  above  the  earth  does  not  vary 
much  from  60  miles.  It  is  rarely  visible  in  the  latitude  of  New 
Orleans,  occasionally  in  the  latitude  of  New  York,  and  rather 
more  frequently  in  the  latitude  of  Quebec;  its  maximum  fre- 
quency is  in  the  latitude  of  Norway  and  the  southern  part  of 
Alaska. 

The  time  of  frequency  varies.  At  Hammerfest,  Norway,  it 
is  not  visible  during  the  summer  months,  presumably  because 
of  daylight.  In  New  York,  the  spring  and  fall  months  are  the 
periods  of  greatest  frequency.  Records  from  1764  show  that 
auroras  are  much  more  frequent  during  the  periods  in  which 
sun  spots  are  most  frequent;  this  is  one  reason  why  the  aurora 
is  thought  to  be  due  to  solar  influence. 

The  work  of  the  observer  is  to  watch  carefully  and  to  note 
faithfully  whatever  is  visible.  Information  is  desired  con- 
cerning the  position,  direction  and  extent  of  the  arch,  if  one 
appears — otherwise  the  position  of  the  patch  or  patches  of  light. 
It  is  desirable  to  know  whether  the  arch  takes  the  form  of  a 
curtain,  a  luminous  band,  or  a  corona.  It  is  also  desirable  to 
note  whether  the  light  occurs  in  rays  with  dark  spaces  between 
them,  or  is  a  diffuse  illumination  without  definite  outlines,  or 
takes  the  form  of  dancing  streaks  of  light,  changing  rapidly  in 
color,  form,  and  intensity.  When  possible,  it  is  well  to  compare 
the  aurora  with  illustrations  in  any  known  publication, 
especially  with  those  in  the  Encyclopaedia  Britannica. 


.12     ATMOSPHERIC   ELECTRICITY:    OPTICAL    PHENOMENA 

Thunder-storms. — The  phenomena  of  thunder-storms  have 
been  known  ever  since  human  beings  peopled  the  earth.  The 
cause  or  causes  are  still  imperfectly  known. 

Thunder-storms  derive  their  name  from  the  reverberations 
and  crashes  of  thunder  following  lightning  discharges,  which 
possess  an  intensity  unknown  except  in  nature.  These  dis- 
charges take  place  between  cloud  and  earth,  between  earth  and 
cloud,  and  between  cloud  and  cloud.  But  the  lightning  dis- 
charges are  not  the  cause  of  the  storm;  they  are  incidents 
merely  in  its  progress;  and  except  in  intensity  and  volume  the 
thunder  does  not  differ  from  the  snapping  of  an  electric  spark. 

Several  things  take  place  in  the  formation  of  a   thunder- 


Afler  Humphreys. 

The  movement  of  the  wind  in  a  thunder-storm;  A,  base  of  cumulo- 
nimbus cloud;  B,  ground  level.  A  roll  scud  forms  between  the  wind  of 
updraught  and  that  of  a  downdraught. 

storm.  A  strong  updraught  of  air  and  the  shattering  of  rain- 
drops are  among  the  features  necessary  to  produce  free 
electricity.  The  updraught  of  air  is  almost  always  a  noticeable 
feature,  and  this  takes  place  conspicuously  in  the  cumulus 
thunder-head.  Ordinarily  the  base  of  the  cumulus  cloud  is 
less  than  I  mile  in  height;  but  the  updraught  that  precedes 
the  thunder-storm,  and  is  a  potent  cause  of  it,  carries  the  cauli- 
flower head  of  the  cloud  to  a  height  of  4  or  5  miles.  It  is  within 
this  head  that  the  potential  electricity  of  the  raindrops  is 
changed  to  kinetic  or  free  electricity. 

Experiments  have  shown  that  a  blast  of  air  driven  against 
drops  of  distilled  water,  with  a  force  sufficient  to  blow  them 
into  spray,  produces  both  positive  and  negative  electricity — 


THUNDER-STORMS  113 

three  times  as  many  negative  as  positive  electrons.1  It  has 
been  found  also  that  a  velocity  of  25  feet  (8  meters)  per  second, 
or  more,  will  cause  the  larger  drops  to  be  shattered  and  beaten 
into  spray.2  That  is,  if  the  drops  falling  in  still  air  reach  a 
velocity  of  25  feet  per  second,  they  will  be  broken  into  smaller 
drops;  or  if  the  updraught  exceeds  25  feet  per  second,  the 
drops  cannot  fall  against  it;  they  will  be  shattered  and 
carried  upward  until  the  velocity  of  the  updraught  is  much 
reduced. 

"Clearly,"  Dr.  Humphreys  states,  "the  updraughts  within  a 
cumulus  cloud  frequently  must  break  up,  at  about  the  same 
level,  innumerable  drops,  which,  through  coalescence,  have 
grown  beyond  the  critical  size  and  thereby  according  to  Simp- 
son's experiments,  produce  electrical  separation  within  the  cloud 
itself.  Under  the  choppy  surges  of  a  thunder-storm,  the  drops 
may  undergo  disruption  and  coalescence  many  times,  and  with 
each  disruption  a  correspondingly  increased  electrical  charge. 
Hence,  once  started,  the  electricity  of  a  thunder-storm  rapidly 
grows  to  a  considerable  maximum.  After  a  time,  the  larger 
drops  here  and  there  reach  places  below  which  the  updraught 
is  slight;  then  they  fall  as  positively  charged  rain.  The  negative 
electrons  in  the  meantime  are  carried  up  into  the  higher  part 
of  the  cumulus  where  they  unite  with  the  particles  of  cloud 
matter  and  thereby  facilitate  their  coalescence  into  negatively 
charged  drops.  Hence  the  heavy  rain  of  a  thunder-storm 
should  be  positively  charged — as  almost  always  it  is — and  the 
gentler  portions  negatively  charged — which  frequently  is  the 
case." 

The  falling  rain — and  also  the  hail  which  occasionally 
attends  a  thunder-storm — cools  the  air  through  which  it  passes 
and  the  cold  air  sinks  to  the  earth  with  a  considerable  velocity. 
As  it  reaches  the  earth  the  down-rush  plows  underneath  the 
warm,  moist  air  in  front  of  the  storm,  lifting  it  and  thereby 
aiding  the  updraught.  As  the  cold  air  spreads  over  the  ground 
its  velocity  is  great  enough  to  raise  clouds  of  loose  dust  that 
almost  always  precede  the  fall  of  rain. 

As  in  the  case  of  other  storms,  the  latent  heat  set  free  by 
the  condensation  of  moisture  is  the  fuel  of  the  thunder-storm, 

1  C.  G.  Simpson,  London  Meteorological  Office. 

2  P.  E.  A.  Lenard. 


114     ATMOSPHERIC  ELECTRICITY:    OPTICAL   PHENOMENA 


THUNDER  115 

and  the  cause  of  the  updraught.  Rapid  evaporation,  on  the 
other  hand,  together  with  the  expansion  of  air  in  the  updraught, 
is  sufficient  to  account  for  the  cold  air,  still  further  chilled  by 
rain  and  hail,  which  finally  culminates  in  the  downrush. 

Practically,  the  cumulus  is  the  parent  of  the  thunder-storm, 
and  when  it  develops  into  the  cumulo-nimbus  stage  it  is  essen- 
tially a  thunder-storm.  Even  the  apparently  quiet  cloud  is  al- 
ways in  motion  within  itself.  Rising  currents  of  moist  air,  chilled 
by  its  own  expansion,  cause  condensation  of  the  vapor  into  cloud 
matter.  The  coalescence  of  cloud  matter  into  mist  and  droplets 
results  in  their  fall  to  a  lower  level,  where  they  are  again  vapor- 
ized; and  the  vapor,  in  turn,  rises  in  the  updraught.  All  this  is 
constantly  changing  and  disturbing  the  electric  potential.  When, 
however,  the  updraught  is  strong  enough  to  shatter  the  drops  into 
mist,  the  potential  becomes  so  high  that  the  violent  discharges 
constitute  the  thunder-storm. 

In  other  words,  if  the  updraught  is  sufficiently  strong  to 
hurl  the  cloud  matter  to  a  height  where  condensation  is  very 
rapid,  and  also  to  shatter  the  falling  rain-drops,  the  cumulus 
develops  into  a  thunder-head  at  the  top  and  a  thunder-storm  at 
the  base. 

Thunder. — The  distance  of  the  discharge  may  be  found 
approximately  by  noting  the  interval  between  the  flash  and  the 
thunder,  allowing  noo  feet  per  second  l  for  the  velocity  of  the 
sound  wave.  In  general,  a  nearby  discharge  is  followed  by  an 
instantaneous  report  and  this  in  itself  indicates  that  the  observer 
is  in  the  danger  zone.  It  also  indicates  a  probability  that  the 
discharge  passed  between  cloud  and  earth  rather  than  between 
cloud  and  cloud.  If  there  is  no  visible  flash,  it  is  likely  that 
the  discharge  took  place  between  cloud  and  cloud;  and  if  no 
thunder  follows  a  discharge,  either  the  discharge  occurred  at 
a  distance  so  great  that  the  sound  wave  became  inaudible,  or 
else  it  was  a  silent  "brush"  discharge. 

The  long-drawn  rolling  of  the  thunder  may  be  due  to  either 
or  both  of  two  causes.  If  the  lightning  is  a  flow  or  "streak" 
a  mile  or  more  in  length,  the  sound  from  the  farther  part  re- 
quires a  proportionately  longer  time  to  reach  the  observer  than 
that  for  the  nearby  part.  Another  factor  also  must  be  con- 
sidered; what  appears  to  be  a  single  discharge  may  be  an 
1  The  rate  varies  slightly  with  temperature  and  density  of  the  air. 


116     ATMOSPHERIC   ELECTRICITY:    OPTICAL   PHENOMENA 

oscillatory  discharge  l  which  does  not  differ,  except  in  intensity, 
from  the  undamped  spark  of  a  wireless  transmitter,  the  several 
oscillations  producing  separate  but  interfering  sets  of  sound 
waves.  A  more  satisfactory  theory  makes  the  extreme  and 
sudden  heating  of  the  air,  with  its  moisture  content  practically 
an  explosion  with  compression  waves  identical  with  those  caused 
by  instantaneous  explosions.  The  reflection  of  sound  also  may 
be  a  factor  in  reverberation.2 

Forms  of  Lightning. — The  most  common  form  of  discharge 
is  shown  in  the  accompanying  illustration.  The  discharge 
merely  follows  the  line  of  least  resistance.  The  zig-zag  discharge, 
with  sharp  angles  and  saw-teeth  points,  once  patronized  by 
artists  in  order  to  give  effect  to  their  illustrative  work,  has  never 
been  discovered  in  photographs  of  lightning  discharges.  The 
most  extraordinary  effects  of  lightning  are  the  dark  flashes 
occasionally  caught  in  photographs  of  lightning. 

Sheet  lightning  is  generally  regarded  as  the  reflection  of  distant 
flashes  from  the  surface  of  clouds.  On  various  occasions  the  ex- 
change of  electricity  takes  the  form  of  a  bluish  glow  between  the 
earth  and  a  low  cloud.  This  form  of  discharge  is  rare;  probably 
it  does  not  differ  from  the  brush-shaped  discharge  visible  when  a 
static  generator  is  operated  in  the  dark.  The  St.  Elmo  fire  is  a 
discharge  of  this  sort.  -  During  its  occurrence,  the  peaks  of  roofs, 
the  limbs  of  trees,  flag-poles,  church  spires,  and  weather  vanes 
are  tipped  with  coronal  circles  of  electricity.  The  St.  Elmo  fire 
is  of  rare  occurrence.  It  sometimes  follows  thunder-storms. 

Ball  lightning  has  been  observed  so  many  times  that  its 
existence  seems  to  be  established  beyond  doubt.3  It  has  been 

1  The  oscillatory  discharge  is  regarded  as  doubtful  by  some  meteorologists. 
At  all  events,  in  traversing  a  conductor  of  moderate  resistance  it  is  damped 
practically  to  a  current  of  unidirectional  character. 

2  The  electrolytic  decomposition  of  water  vapor  and  its  recomposition 
in  the  form  of  successive  explosions  also  has  been  suggested. 

3  Mr.  George  Reeder  and  his  assistant  Mr.  Seaton  of  the  Weather  Bureau 
Station,  University  of  Missouri,  describe  an  instance  of  ball  lightning,  as 
"a  pale  red,  slightly  corrugated  ball,  apparently  about  2  inches  in  diameter, 
moving  across  a  space  of  about  6  feet  between  the  telephone  and  a  window. 
The  ball  seemed  to  float  as  a  liquid  bubble  does,  though  it  seemed  solid. 
It  kept  a  fairly  straight  line  for  the  window;    it  rolled  over  the  window  sill 
and  disappeared — not  into  the  outer  air,  but  by  flickering  out  as  a  bubble 
does.     There  was  no  explosion  or  sound  of  any  kind  except  a  click  of  the 
telephone;    there  was  no  odor  nor  mark  of  any  kind  on  the  window  sill." 


OCCURRENCE  OF  THUNDER-STORMS  117 

explained  as  being  due  to  a  slowly  moving  point  at  which  in- 
tense discharge  is  taking  place;  but  this  explanation  is  merely 
a  possibility,  not  an  established  fact. 

Occurrence  of  Thunder-storms. — Roughly  speaking,  the 
lower  the  latitude  of  moist  regions,  the  greater  the  frequency 
of  thunder-storms.  In  general,  of  two  regions  of  the  necessary 
warmth,  one  having  moist  air  and  the  other  dry  air,  the  former 
is  more  likely  to  be  visited  by  thunder-storms.  They  are  more 
prevalent  in  the  United  States  than  in  Europe;  they  are  more 
prevalent  in  the  southern  part  of  the  United  States  than  in  the 
northern  part,  so  far  as  the  region  east  of  the  Rocky  Mountains 
is  concerned. 

According  to  A.  J.  Henry  of  the  United  States  Weather 
Bureau,  the  regions  of  greatest  frequency  are  in  Florida,  where 
thunder-storms  occur  45  days  in  the  year;  in  the  central  Mis- 
sissippi Valley,  where  they  occur  35  days  in  the  year;  and  in 
the  upper  Missouri  Valley,  where  the  average  is  30  days  in  the 
year.  Thunder-storms  rarely  occur  in  the  Pacific  Coast  states, 
but  they  are  common  in  the  Plateau  Region. 

Practically  all  the  violent  thunder-storms  of  the  United  States 
occur  in  the  warm  months.  By  far  the  greater  number  occur 
in  June,  July  and  August,  during  the  hottest  part  of  the  day. 
There  is  also  a  period  of  minor  frequency  between  9  o'clock 
at  night  and  midnight.  Over  the  sea,  however,  the  period  of 
frequency  is  apt  to  be  in  the  early  morning,  before  daylight. 

Occasionally  the  updraught  of  the  ordinary  cyclone  may 
produce  a  thunder-storm;  the  thunder-storms  of  winter  are  of 
this  sort  and  they  are  rarely  severe. 

Pressure  Waves. — The  accompanying  barogram,  recorded 
at  the  Mount  Vernon  Meteorological  Laboratory,  illustrates 
pretty  clearly  the  progress  of  a  thunder-storm.  The  barometer 
had  fallen  steadily  for  more  than  twelve  hours  preceding  the 
storm;  and  this  continued  until  well  along  in  the  afternoon. 
The  slight  rise  of  the  barometer  in  the  morning  is  the  diurnal 
pressure  wave.  The  jump  in  pressure  in  the  afternoon  is  the 
characteristic  "thunder-storm  nose"  which  usually  is  found  on 
barograph  records  of  thunder-storms.  An  expert  observer  does 
not  need  to  refer  to  his  daily  reports  to  find  the  records  of 
thunder-storms;  the  barograms  show  them  in  rnost  instances. 
The  rise  in  pressure  occurs  when  the  descending  wind  lifts 


US     ATMOSPHERIC  ELECTRICITY:    OPTICAL  PHENOMENA 


the   warmer   air  above   it.      A   second    "nose"   appears   about 

9.00,  when  clearing  gusts 
marked  the  end  of  the 
storm. 

Forecasting  Thun- 
der-storms.— From    the 
nature  of  the  case,    the 
general   forecasts    made 
by  the  Weather  Bureau 
cannot  designate  the  loci 
of      possible      thunder- 
A  thunder-storm  nose.     Barogram  of  Mount     storms,  because  the  gen- 
Vernon    Meteorological  Laboratory,  April     eral   forecasts  are   macje 
21,1917.     The  rise  in  pressure  at  6:15  p.m.  f          11  t      i 

,.  A       A       u      •  u-    *u      to°  *ar  ahead,  and  also 

was  caused  by  the  downdraught  within  the 

cumulo-nimbus  thunder-head,  because  such  storms  are 

local. 

The  meteorologist  in  charge  of  the  local  station  is  able 
to  forecast  more  definitely;  and,  where  stations  not  far  apart 
are  fortunately  situated,  the  formation  of  thunder-storms  may 
be  indicated  with  a  fair  probability  of  verification.  With 
warm,  moist  air  on  the  south  side  of  a  low,  thunder-storms  may 
be  expected;  and  if  one  has  formed,  its  path  may  be  predicted 
with  reasonable  exactness.  In  the  hands  of  a  trained  observer 
a  barograph  is  a  most  useful  aid.  With  the  aid  of  the  daily 
weather  map,  the  local  conditions  of  temperature  and  hu- 
midity, and  the  barogram,  at  least  two  hours'  notice  may  be 
given. 

The  layman  also  may  forewarn  himself  with  a  reasonable 
degree,  if  not  of  certainty,  at  the  least,  of  probability.  An 
aneroid  barometer,  if  watched  closely,  may  be  serviceable; 
unless  intelligently  used  it  is  of  doubtful  service  to  any  but  a 
trained  observer.  Nevertheless,  there  are  indications  that 
should  warn  even  a  casual  observer  who  bears  in  mind  that 
the  thunder-storms  disastrous  to  crops  occur  mainly  in  June, 
July,  and  August,  and  also  that  almost  always  they  occur 
between  mid-afternoon  and  sunset. 

Warm  and  moist  air  is  necessary  to  the  formation  of  a 
thunder-storm ;  moderately  quiet  air  is  also  necessary.  A  thun- 
der-storm is^  not  likely  to  form  where  a  stiff  wind  is  blowing. 
Cumulus  clouds  may  be  regarded  with  suspicion;  indeed  the 


SAFEGUARDS  AGAINST  LIGHTNING  119 

cumulus  is  the  thunder-storm  factory;  and  when  it  develops  into 
a  cumulo-nimbus,  the  thunderstorm  is  probably  at  hand. 

If  the  air  of  a  warm,  moist  summer  afternoon  becomes  still 
and  oppressive  and  if  cumulus  clouds  increase  in  size,  a  thunder- 
storm is  very  likely  to  follow ;  and  if  a  nearby  cumulus  expands 
vertically  into  a  thunder-head  the  storm  is  pretty  certain  to 
follow,  somewhere  or  other  in  the  vicinity.  The  thunder-head 
may  be  visible  every  where  within  a  radius  of  25  miles,  but  the 
storm  path  may  be  a  narrow  strip  not  more  than  30  miles  in 
length.  The  path  of  the  thunder-storm,  like  that  of  the  tornado, 
is  determined  by  the  circulation  of  the  cyclone  in  which  it  is 
formed.  Its  forward  movement,  except  in  the  extreme  southern 
part  of  the  United  States,  is  from  a  westerly  to  an  easterly 
direction. 

Safeguards  Against  Lightning. — The  destructive  effects  of 
lightning  in  the  United  States  are  chiefly  loss  of  life  and  loss  from 
fire.  Loss  of  life  occurs  usually  when  lightning  strikes  trees 
under  which  people  and  animals  have  taken  shelter.  Trees 
are  the  objects  most  frequently  struck.  Wooden  buildings  when 
struck  are  apt  to  take  fire  instantly,  but  cases  are  on  record 
which  show  that  wet  shingles  and  weather  boards  may  be 
ripped  off  without  further  damage.  Among  structures,  oil  tanks 
stand  first  in  the  likelihood  of  destruction  by  lightning.  Church 
spires  and  large  barns  are  frequently  struck,  and  isolated  build- 
ings are  regarded  as  a  far  greater  risk  than  city  buildings; 
indeed,  in  the  compactly  built  areas  of  a  city  the  risk  from 
lightning  stroke  is  negligible. 

Lightning  rods  afford  the  best  protection  against  lightning. 
J.  Warren  Smith  of  the  United  States  Weather  Bureau  found 
that  in  many  thousand  insurance  risks,  the  destruction  of 
rodded  buildings  was  negligible.  Other  authorities  regard  the 
safety  afforded  by  lightning  rods  at  from  90  per  cent  to  97 
per  cent.  The  Bureau  of  Standards  1  points  out  the  necessity  of 
connecting  all  exposed  metal  surfaces  such  as  metal  roofs, 
gutters  and  tanks  with  the  lightning  rods.  Sir  Oliver  Lodge 
recommends  iron  in  preference  to  copper  as  a  material  for 
lightning  rods  for  the  reason  that  its  greater  resistance  tends 
to  damp  the  oscillations  of  a  discharge,  practically  converting 
them  into  a  one-way  current. 

1  Bulletin  56. 


120     ATMOSPHERIC  ELECTRICITY:    OPTICAL   PHENOMENA 

ATMOSPHERIC  OPTICAL  PHENOMENA 

A  ray  of  light  may  pass  through  a  solid,  such  as  glass;  a 
liquid,  such  as  water;  or  a  gas,  such  as  oxygen,  nitrogen  or  water 
vapor — that  is,  the  air — without  much  apparent  loss.  Such 
substances  are  transparent.  In  passing  through  different  sub- 
stances the  ray  is  likely  to  be  bent  out  of  its  course,  as  is  appa- 
rent when  a  stick  is  thrust  obliquely  into  a  body  of  water. 
The  bending  of  the  ray  is  called  refraction.  Or  if  it  is  turned 
back,  as  when  it  impinges  upon  a  mirror,  it  is  said  to  be  reflected. 

A  ray  of  light  impinging  upon  a  piece  of  black  cloth  is  said 
to  be  absorbed.  If  only  a  part  of  the  ray  is  absorbed,  the  rest 
being  reflected,  the  parts  of  the  ray  reflected  produce  the  sensa- 
tion of  color. 

If  a  ray  of  light  is  passed  through  a  wedge-shaped  prism 
the  component  parts  of  the  ray  are  unequally  bent  or  refracted, 
and  reach  the  eye  in  a  series  of  colors.  Red  is  the  least  re- 
fracted; violet  suffers  the  greatest  refraction.  A  ray  of  white 
light,  therefore,  is  not  of  a  "bundle"  of  wave-lengths  of  the 
same  magnitude,  but  a  bundle  of  an  infinite  number  of  rays  of 
different  wave-lengths. 

In  passing  by  the  edge  of  an  opaque  body,  or  in  passing 
through  a  very  narrow  slit,  a  ray  of  light  is  deflected  slightly, 
and  alternate  fringes  of  light  and  dark  bands  are  produced. 
The  deflection  and  interference  constitute  diffraction,  and  dif- 
fraction is  also  a  factor  in  giving  various  color  tints  to  the  sky. 

The  various  atmospheric  optical  effects  of  the  sky  are  pro- 
duced mainly  by  refraction,  reflection,  diffraction,  and  ab- 
sorption of  light  by  the  constituents  of  the  air.  The  color  of  the 
sky  itself  is  due  to  the  irregular  scattering  and  dispersion  of  light 
as  the  sun's  rays  glance  from  the  gaseous  molecules  and  minute 
dust  motes  of  the  air.  The  most  common  incidents  of  atmospheric 
optical  phenomena  are  coronas,  halos,  rainbows  and  mirages. 

Corona. — A  corona  consists  of  a  ring — sometimes  several 
rings — rarely  more  than  4  degrees  of  arc  measurement  in  di- 
ameter, surrounding  the  sun  or,  more  commonly,  the  moon. 
The  corona  is  a  case  of  diffraction,  the  deflection  of  rays  passing 
by  water  droplets.1  The  inner  border  of  the  ring  is  brownish- 

1  In  a  foggy  atmosphere,  an  observer  with  his  back  to  the  sun  sometimes 
sees  a  dim,  colored  ring  surrounding  his  shadow  which  is  cast  upon  the  fog. 
This  phenomenon,  known  as  a  "glory  halo,"  is  probably  a  corona. 


OPTICAL  PHENOMENA:  HALO 


121 


red.  Within  the  ring  is  a  bluish-white  surface,  the  aureole. 
If  spectrum  colors  other  than  the  red  are  observable,  they 
follow  each  other  in  order  from  violet  to  red,  reversing  the  order 
of  halo  colors.  This  sequence  of  color  sometimes  is  repeated 
several  times  in  the  case  of  the  corona  but  not  with  the  halo. 

Halo. — The  most  common  form  of  halo  is  the  ring  around 
the  sun  or  moon.  It  has  a  radius  of  about  22  degrees  of  a  great 
circle.  At  times,  however,  the  halo  is  a  complex  arrangement  of 
concentric  tangential  and  independent  arcs  of  circles.  The  sim- 


from  a  drawing  made  by  himself. 
Lunar  halo  observed  by  Gen.  A.  W.  Greely  at  Fort  Conger. 

pie  halo  is  practically  a  rainbow,  red  inside  the  ring,  with  colors 
on  the  outer  side  ranging  in  spectrum  order.  Unless  the  halo 
is  strong,  however,  the  impression  to  a  casual  observer  is  that 
of  a  white  ring.  Occasionally  another  fainter  and  incomplete 
ring  of  46  degrees  radius  may  be  observed.  Still  more  rarely 
a  white  ring  parallel  to  the  horizon  and  passing  through  the 
sun  is  observable.  At  or  near  the  intersection  of  this  circle 
with  the  halo,  mock  suns,  parhelia,  or  mock  noons,  paraselenes, 
appear  as  very  bright  spots,  with  red  predominating.  Mock 
suns  and  mock  moons  are  seen  at  times  in  other  positions.  The 


122     ATMOSPHERIC  ELECTRICITY:    OPTICAL  PHENOMENA 

mock  suns  at  the  intersection  of  the  22-degree  circle  are  usually 
bright  and  decidedly  red  next  the  sun ;  those  at  the  intersection 
of  the  46-degree  circle  are  faint.  Occasionally  a  white  spot  is 
observed  on  the  sky  opposite  the  sun.  This  is  the  counter  sun, 
or  ant-helion. 

As  a  rule,  the  various  circles,  with  the  exception  of  the  halo 
circle,  are  only  partly  visible;  and  in  many  cases  the  unusual 
arcs  seem  to  have  no  connection  with  the  halo.  Many  interest- 
ing illustrations  of  complex  halo  circles  have  been  published. 
Usually  these  have  the  circles  of  22  degrees,  46  degrees,  and  the 
mock-sun  circle  in  common;  otherwise  they  are  unlike. 

Occasionally  a  vertical  column  of  sheen  extends  above  and 
below  the  sun — perhaps  more  frequently  the  moon;  it  is  known 
popularly  as  the  pillar  of  light.  Rather  infrequently  a  hori- 
zontal bar  of  sheen  may  be  seen  forming  the  popularly  named 
"heavenly  cross."  l  Sun  pillars,  varying  in  color  from  white  to 
red  are  occasionally  seen  at  sunset  or  at  sunrise.  Patches  of 
color  occasionally  are  observed  in  cirrus  and  cirro-stratus 
;  clouds  at  a  considerable  angular  distance  from  the  sun.  They 
"may  be  due  to  causes  similar  to  those  which  produce  halos, 
but  the  causes  are  not  known. 

Cirrus  or  cirro-stratus  clouds,  or  ice  mist,  in  front  of  the 
sun  or  the  moon  are  necessary  to  the  production  of  halos.  Some 
of  the  ice  crystals  are  tabular;  others  are  columnar  and  pris- 
matic in  shape.  It  is  thought  that  both  reflection  and  refraction 
of  light  are  involved,  each  depending  on  the  character  of  the 
crystals.  Spectrum  colors  which  abound  in  halo  phenomena  are 
explainable  as  a  result  of  refraction ;  white-light  surfaces  may 
be  due  to  reflection. 

Rainbow. — The  rainbow  against  a  dark  gray  background  of 
cloud  is  one  of  the  most  beautiful  objects  in  nature.  It  may 
be  seen  as  a  full  circle  against  spray  thrown  into  the  air,  or 
against  a  mist.  The  rainbow  of  the  summer  shower  consists 
of  a  bright  arc  near  the  norizon  and  usually  a  fainter  arc  above. 
The  radius  of  the  bright,  or  primary  bow  is  about  42  degrees  of 
arc ;  that  of  the  upper  or  secondary  bow  is  not  far  from  52  degrees 
of  arc. 

1  This  effect  may  be  produced  by  looking  at  the  moon  through  a  piece  of 
polished  copper  screen-netting  held  at  a  distance  of  20  feet.  It  is  an  effect 
of  diffraction. 


OPTICAL  PHENOMENA:  MIRAGE 


123 


The  rainbow  is  best  observed  when  the  sun  is  not  more  than 
45  degrees  above  the  horizon;  it  forms  in  the  side  of  the  sky 
opposite  the  sun.  On  rare  occasions  a  tertiary  bow  may  be  seen 
between  the  observer  and  the  sun. 

The  colors  of  the  rainbow  vary  in  intensity  and  in  quality. 
Red  is  always  in  evidence  outside  the  primary  and  inside  the 
secondary  bow;  orange,  yellow  and  green  are  commonly  though 
faintly  observable;  blue  is  sometimes  seen;  but  violet  is  rarely 
if  ever  observable.  The  strength  and  the  sequence  of  the  colors 
depends  mainly  on  the  size  of  the  drops,  but  partly  on  their 
distance  and  the  number  of  them. 

Each  observer  sees  his  own  rainbow,  and  each  rainbow  is 


Refraction  of  light  passing  through  rain-drops. 

practically  a  series  of  hollow  concentric  cones,  the  vertex  being 
at  the  eye  of  the  observer.  The  rainbow  moves  forward,  back- 
ward or  sideways  as  the  observer  moves.  A  shower  in  one 
part  of  the  sky  and  sunshine  in  another,  the  observer  being 
between,  are  requisite  for  rainbow  formation;  and  this  condi- 
tion, in  most  parts  of  the  world,  is  confined  to  summer  showers. 
Mirage. — Owing  to  changes  in  temperature  the  density  of  the 
air  varies  almost  constantly  at  different  heights.  Rays  of  light 
passing  through  air  of  varying  density  are  bent  differently 
with  each  change  of  density.  An  observer  looking  at  a  distant 
object  sees  the  object  with  distorted  outlines.  An  elliptical 
sun  at  sunset  is  very  common;  and  sometimes  one  sees  it  with 
greatly  distorted  outlines. 


124     ATMOSPHERIC  ELECTRICITY:    OPTICAL  PHENOMENA 

When  a  layer  of  air  rests  quietly  on  another  the  plane  of 
contact,  if  below  the  observer,  reflects  the  sky  in  much  the 
same  manner  as  does  a  body  of  water.  An  object  at  this  plane 
is  seen  both  upright  and  inverted,  thereby  forming  a  mirage. 
If  the  plane  of  contact  is  materially  above  the  eye  of  the  ob- 
server the  inversion  occurs  in  the  air.  Occasionally  inverted 
images  of  the  shipping  in  the  harbor  are  formed.1 

The  looming  of  objects — that  is,  bringing  to  sight  objects 
that  normally  are  below  the  horizon — is  clearly  a  case  of  re- 
fraction. The  rays  of  light  which  should  pass  above  the  observer 
are  bent  within  reach  of  his  vision. 

According  to  legends  a  fairy  named  Morgana  hovered  around 
and  about  the  southern  coasts  of  Italy.  This  sprite  used  her 
powers  romantically  rather  than  maliciously  to  change  the  com- 
monplace shoreline  across  the  straits  of  Messina  to  a  most 
wonderful .  landscape  of  turreted  embattlements  and  castellate 
fortifications.  Hence  the  name,  Fata  Morgana.  The  phenom- 
enon apparently  is  produced  by  a  horizontal  layer  of  air,  denser 
at  the  center  than  at  its  surface.  It  therefore  becomes  practically 
a  cylindrical  lens  which  magnifies  in  a  vertical  but  not  in  a 
horizontal  direction.  It  may  be  considered  as  a  form  of  looming. 

It  is  probable  that  the  Brocken  spectre  is  produced  in  part 
by  a  mass  of  air  which  acts  as  a  lens.  The  traditional  "heiligen- 
schein,"  or  halo,  is  an  example  of  diffraction,  however. 

1  Many  physicists  hold  that  the  inversion  in  such  instances  is  due  to 
refraction.  In  one  popular  textbook  of  physics  a  diagram  graphically 
describes  the  method  of  refraction;  but  the  diagram  illustrates  reflection. 
The  mirage  is  considered  in  detail  in  Chapter  XII. 


CHAPTER  XI 
THE  DUST  CONTENT  OF  THE  AIR 

Dust  is  usually  classed  as  "foreign  matter  of  the  air."  Such 
a  view  of  the  shell  of  wind-blown  dust  is  permissible.  It  might 
also  be  considered  logical  as  regards  the  finer  dust  particles 
which  do  not  reach  the  ground  except  by  means  other  than  their 
own  gravity.  It  is  hardly  logical  to  consider  the  dust  of  the 
stratosphere  as  foreign  matter  of  the  air;  for,  as  a  matter  of 
fact,  it  is  permanently  and  not  temporarily  there.  Moreover, 
cosmic  dust  seems  to  pervade  every  part  of  the  universe  which 
the  solar  system  traverses,  and  the -earth  is  constantly  gathering 
dust  from  space. 

"Invisible"  Dust;  Characteristics. — Practically  nothing  is 
known  of  the  dust  of  the  stratosphere,  except  that  its  presence 
is  revealed  in  various  ways.  The  particles  themselves  are  too 
small  to  be  discerned  by  any  mechanical  method  at  present 
known.  En  masse  they  reflect  enough  sunlight  to  reveal  their 
existence,  but  not  their  form  nor  their  constitution.  In  part, 
and  probably  to  a  great  extent,  they  constitute  the  overhead 
effect  noted  by  observers  for  more  than  six  thousand  years — 
the  sky.  An  estimate  of  the  size  of  such  dust  particles  cannot 
be  made  with  any  degree  of  accuracy.  It  is  safe  to  say  that 
they  are  much  smaller  than  the  smoke  particles  that  escape 
from  the  burning  end  of  a  cigar.  It  safe  also  to  say  that  they 
are  smaller  than  the  particles  which  constitute  the  "blue  haze." 
As  a  matter  of  fact,  rapid  changes  in  sky  polarization  indicate 
about  the  only  thing  that  can  be  authoritatively  asserted — 
namely,  that  the  invisible  particles  behave  much  like  the  mole- 
cular constituents  of  the  air. 

Measurements. — Under  the  stratosphere,  the  dust  particles 
of  the  air  are  of  every  possible  size,  from  those  of  the  blue  haze 
to  the  coarse  rock  waste  of  the  simoon.  The  micromillimeter, 
practically  the  twenty-five  thousandth  part  of  an  inch,  is  a 
convenient  unit  of  measurement.  It  is  convenient  because  of 

125 


126  THE   DUST   CONTENT  OF   THE  AIR 

the  fact  that  dust  particles  of  this  dimension  are  very  near  to 
the  size  of  the  permanently  floating  dust  motes  of  the  air. 

The  research  work  of  Dr.  John  Aitkin,  the  highest  authority 
on  the  subject,  has  shown  that  clean  air  contains  from  3000 
to  5000  visible  dust  particles  per  cubic  inch.  The  air  of  school- 
rooms and  public  buildings  with  undressed  wood  floors  carries 
from  60,000  to  80,000  particles  which  are  visible  under  the 
high  power  of  a  microscope,  and  an  unknown  number  which 
can  be  counted  only  when  amplified  in  size  by  the  condensation 
of  moisture  upon  their  surface.1  Dr.  Aitkin  found  the  cleanest 
air  at  snow-clad  heights  in  the  Alps,  and  not  over  the  sea,  as 
one  might  expect. 

Electrification. — To  the  best  of  knowledge,  the  invisible 
dust,  both  in  the  stratosphere  and  the  sphere  of  convection, 
does  not  depend  on  winds  for  its  distribution.  The  particles 
themselves  behave  as  do  other  ionized  bodies,  and  it  is  not 
impossible  that  their  suspension  in  the  air  is  due  to  electrifica- 
tion.2 There  seems  to  be  no  reason  why  the  ionization  of 
minute  dust  particles  should  not  occur  in  the  same  manner 
as  the  ionization  of  the  molecular  constituents  of  the  air. 

1  The  dust-counter  used  by  Dr.  John  Aitken  consisted  of  a  chamber  or 
receiver,  into  which  a  measured  portion  of  air  was  drawn.     The  receiver 
contained  a  small  amount  of  water — enough  to  keep  the  air  pretty  nearly 
at  saturation.     A  slight  reduction  of  temperature  by  means  of  an  air  pump 
causes  almost  instant  condensation.     By  counting  the  droplets  condensed 
on  a  ruled  silver  plate  within  the  receiver,  using  a  magnifying  lens  therefor, 
the  number  of  droplets  per  cubic  centimeter,  or  per  cubic  inch,  may  be  esti- 
mated.    Dr.  Aitken  obtained  the  best  results  when  the  dust  content  of  the 
air  was  small.     In  practice  he  therefore  mixed  the  air  to  be  examined  with 
a  measured  quantity  of  air  made  dustless  by  nitration.     A  modified  dust- 
counter,   the   "koniscope"   is  a   more   practical   instrument,   though  not   so 
accurate. 

2  A  solid  of  i  inch  cubic  measurement,  weighing  I  ounce,  has  6  square  inches 
of  surface.     If  it  be  shaved  into  slices  one  one-thousandth  of  an  inch  in 
thickness,  each  slice  loses  999  parts  of  the  original  weight  but  only  a  little 
more  than  4  parts  of  the  original  surface.     That  is,  in  subdivision,   a  sub- 
stance loses  weight  much  more  rapidly  than  surface.     The  weight  of  a  dust 
particle  one  twenty-five  thousandth  part  of  an   inch  in   dimension  is  less 
than   one  fifteen  trillionth   of  an   ounce.     The   surface   is   almost   infinitely 
great  in  comparison.     Now,  the  electric  charge  of  a  dust  particle,   condensed 
on  its  surface,  is  of  the  same  kind  as  that  of  the  earth.     Therefore  they 
mutually  repel.     It  is  only  fair  to  add  that  the  theory  of  the  electrification 
of  dust  is  not  fully  substantiated. 


DUST  AND  CONDENSATION  127 

Dust  and  Condensation. — The  experimental  work  of  Dr. 
Aitkin  showed  conclusively  that  the  moisture  of  the  air  con- 
densed with  difficulty  in  dustless  air  even  when  the  temperature 
was  several  degrees  below  saturation;  in  normal,  or  dust-laden 
air,  condensation  took  place  readily.  The  repetition  of  Dr. 
Aitkin's  experiments  under  widely  diverse  conditions  has  left 
no  doubt  that  the  dust  particles  of  the  air,  including  sulphur 
gases  set  free  by  the  combustion  of  fuel,  are  the  most  important 
factors  in  condensation.  The  research  of  C.  T.  R.  Wilson 
brought  to  light  additional  knowledge;  Wilson  found  that 
the  passage  of  a  beam  of  ultra-violet  light  through  air  caused 
condensation,  even  when  its  temperature  was  slightly  below 
that  of  saturation.  Saturation  temperature,  however,  is  not 
wholly  necessary  to  condensation;  a  certain  but  small  amount 
of  condensation  goes  on  below  the  temperature  of  saturation. 
Condensation  goes  on  more  freely  when  the  humidity — both 
absolute  and  relative — is  high. 

Dust  particles  differ  greatly  as  nuclei  of  condensation; 
they  may  be  "good,"  "indifferent,"  or  "poor."  The  reason 
for  the  difference  is  not  known  with  certainty.  It  may  be 
that  quickly  cooling  particles  are  better  condensers  than  slowly 
cooling  particles;  it  may  be  that  a  high  degree  of  ionization 
favors  rapid  condensation;  or  that  the  more  hygroscopic  a 
particle  the  more  freely  it  condenses.  Each  is  a  reasonable 
hypothesis  that  remains  to  be  substantiated. 

Barus  and  Pierce  have  shown  that  the  dust  particles  over 
Providence,  a  manufacturing  center,  are  far  more  favorable 
to  condensation  than  those  observed  contemporaneously  at 
Block  Island.  The  reason  therefor  may  be  a  difference  in  the 
chemical  character  of  the  dust  particles;  it  may  be  due  to  a 
difference  in  the  degree  of  ionization ;  it  may  be  due  to  other  and 
unknown  causes. 

One  thing,  however,  is  certain:  The  dust  particles  belched 
from  the  stacks  of  manufacturing  districts  are  such  excellent 
nuclei  of  condensation  that  the  prevalence  of  fogs  over  such 
districts  has  given  rise  to  the  term  "city  fogs,"1  as  distinguished 

1  It  has  been  pointed  out  that  sulphur  dixoide  molecules  in  themselves 
are  not  "good"  nuclei.  Sulphur  dioxide  is  a  gas  and  is  not  to  be  included 
in  the  dust  content  of  the  air.  But  the  intense  heat  of  combustion  has 
separated  it  from  the  combination  in  which  it  existed.  The  chemical  affinity 


128  THE   DUST   CONTENT   OF   THE   AIR 

from  the  ordinary  advection  fogs.  The  distinction  is  a  practical 
one.  It  is  pertinent  to  add  also  that  a  city  fog  forms  usually 
under  a  lid.  But  while  the  city  fog  condenses  on  particles  that 
are  hygroscopic,  the  fogs  of  swamp  lands,  rivers,  and  ponds 
condense  on  particles  that  are  materially  different.  Con- 
densation does  not  take  place  so  readily — in  other  words,  the 
dust  particles  are  indifferent  nuclei.  A  thick  fog  condensed 
on  nuclei  of  an  indifferent  sort  may  be  "eaten  up"  by  a  slight 
rise  in  temperature;  it  may  rain  itself  to  pieces  by  a  drop  in 
temperature. 

Sources  of  Atmospheric  Dust. — Aside  from  the  dust  picked 
up  and  carried  by  the  winds,  there  are  well-defined  sources 
of  floating  dust  that  must  be  considered.  Cosmic,  or  meteoric 
dust,  is  not  born  of  the  earth ;  it  is  gathered  by  the  earth  from 
space.  Large  particles  fall  to  the  earth;  but  those  materially 
less  than  a  micromillimeter  constitute  the  floating  dust  of  the 
air.  The  character  of  this  dust  can  be  recognized  only  when 
the  particles  fall  to  the  earth  or  are  trapped  while  floating  near 
to  its  surface. 

Many  of  the  particles  thus  caught  are  tiny  meteorites. 
These,  in  many  instances,  are  metallic  globules  or  floating  metal 
bubbles.  They  are  essentially  different  from  the  metallic 
particles  of  smeltery  dust,  emery-wheel  dust,  and  brake-shoe 
dust,  which  also  are  metallic.  The  cosmic  dust  of  non-metal 
character  cannot  be  recognized  with  any  degree  of  certainty; 
indeed,  recognition  of  any  sort  of  dust  whose  particles  are  less 
than  a  micromillimeter  is  difficult.  The  gathering  of  cosmic 
dust  seems  to  be  constant  rather  than  sporadic. 

Additions  of  volcanic  dust  to  the  floating  dust  of  the  air 
are  made  irregularly,  but  they  come  in  enormous  quantities. 
Much  of  the  ash  l  falls  to  the  ground,  but  a  very  large  part 
consists  of  particles  fine  enough  to  constitute  floating  matter. 
The  eruption  of  Krakatoa,2  in  1883,  projected  so  much  floating 

of  the  nascent  gas  is  strong,  and  in  the  air  it  is  apt  to  enter  into  combination 
again  with  dust  particles  of  basic  character,  the  resulting  combination  forming 
nuclei  favorable  to  rapid  condensation. 

1  Volcanic  "ash"  is  not  a  product  of  combustion.     It  is  the  convenient 
name  applied  to  lava  blown  into  fine  dust  by  the  expansive  force  of  steam, 
or  by  other  forces. 

2  The  eruption,  which  threw  the  ash  into  the  air,  had  proceeded  for  several 


SOURCES  OF  ATMOSPHERIC  DUST  129 

dust  into  the  air  that  the  trail,  which  girdled  the  earth  several 
times,  was  visible  at  sunset  for  nearly  two  years.  The  blood- 
red  sky  1  at  times  rivaled  the  northern  lights.  Less  marked  red 
sunsets  followed  the  eruptions  of  La  Soufriere  and  Mont  Pelee 
in  1902.  The  explosion  of  a  great  quantity  of  munitions  in 
New  York  Harbor  was  followed  for  several  days  by  red  sunsets 
observable  as  far  west  as  the  Weather  Bureau  station  at  Ithaca, 
N.  Y.  The  dust  mantle  of  the  Greenland  glacier  is  apparently 
of  volcanic  origin.  Indeed,  volcanic  dust  is  always  an  important 
constituent  of  the  floating  dust  of  the  air;  at  times  it  is  the 
chief  constituent. 

The  floating  dust  of  the  air  has  a  marked  effect  upon  its 
temperature.  Benjamin  Franklin  noted  this  fact.  During 
several  months  in  1783,  the  air  was  filled  with  floating  volcanic 
'dust.  "The  sun's  rays  were  indeed  rendered  so  faint  in  passing 
through  it  that,  when  collected  in  the  focus  of  a  burning  "glass, 
they  would  scarcely  kindle  brown  paper."  The  heating  power 
of  the  sun  was  so  feeble  that  freezing  temperatures  began  nearly 
a  month  before  their  normal  occurrence.  "Delaware  River 
was  closed  in  November  and  remained  ice-bound  until  late  in 
March."  2 

The  years  1812-1816  were  years  of  great  volcanic  activity, 
and  the  air  was  loaded  with  floating  dust.  As  a  result,  the 
year  1816  has  gone  into  history  as  the  "year  without  any  sum- 
mer"— the  year  of  "eighteen  hundred  and  froze- to-death."  In 
Vermont  snow  fell  and  frosts  occurred  every  month  of  that 
year.  On  the  8th  of  June,  snow  on  the  uplands  was  5  or  6 
inches  deep.3 

Humphreys    has    shown    that,   with   a  blanket  of  volcanic 

days,  during  which  time  the  coarser  dust  fell  on  the  nearby  islands  and  into 
the  sea.  This,  a  normal  eruption,  was  separate  and  distinct  from  the  explo- 
sion which  shattered  the  island. 

1  By  reflected  light,  fine  dust  particles  tend  to  a  whitish  color,  and  to  a 
bluish  tint  if  very  fine  and  fewer  in  number.     The  purity  of  the  tint  depends, 
to  a   certain   degree,   on   the   size   of   the   particles.     By  transmitted   light, 
especially  when  the  sun  is  near  the  horizon,  the  blue  and  the  violet  rays  are 
absorbed  and  scattered  and  the  red  rays  reach  the  eye  of  the  observer.     When 
the  air  is  full  of  floating  dust,  the  scattering  of  blue  and  violet  rays  is  very 
great. 

2  The  Philadelphia  Inquirer. 

3  Thompson's  History  of  Vermont. 


130  THE   DUST   CONTENT  OF   THE  AIR 

dust  in  the  air,  while  the  earth  is  receiving  a  lessened  amount 
of  heat  from  the  sun  it  is  radiating  into  space  about  thirty  times 
as  much.1 

The  products  of  combustion  must  also  be  taken  into  con- 
sideration as  having  a  similar  effect  on  absorption  and  radiation 
of  heat.  The  world's  fuel  consumption  each  year  is  the  equiva- 
lent of  about  1,500,000,000  tons  of  coal.  Forest  fires  and  grass 
fires  add  to  the  total  of  combustion  whose  products  in  part 
escape  into  the  air.  By  their  means  an  enormous  number  of 
dust  particles  are  projected  into  the  air  and  distributed  through 
it.  As  a  rule,  the  dust  particles  of  fuel  combustion  are  nuclei 
favorable  to  condensation.  One  cannot  estimate  even  broadly 
the  extent  of  air  pollution  from  this  source;  it  can  be  measured 
chiefly  in  terms  of  city  fogs. 

The  suspended  matter  of  combustion  products  has  been 
measured  at  times.  Systematic  measurements  both  of  suspended 
matter  and  of  matter  which  is  brought  to  the  ground  by  rain- 
fall have  been  made  in  various  parts  of  England,  at  regular 
stations.  The  insoluble  matter  caught  in  gauges  consisted 
chiefly  of  smoke  carbon,  a  mixture  of  free  carbon  and  heavy 
hydrocarbons,  minute  globules  of  liquid  tar  and  insoluble  ash. 
The  soluble  matter  consisted  of  various  sulphates,  chlorine, 
ammonia,  and  soluble  ash.  The  amount  varied  from  a  few 
hundred  tons  per  year  on  each  square  mile  to  nearly  6000  tons 
per  square  mile.  Measurements  in  several  manufacturing  dis- 
tricts of  Pennsylvania  showed  an  average  of  about  1900  tons 
per  square  mile  per  year  falling  to  the  ground.2 

In  regions  where  smokeless  fuel  is  used  there  practically  is 
no  smoke  problem,  and  the  pollution  of  the  air  is  confined 
almost  wholly  to  wind-blown  dust  and  to  local  sources  of 
pollution.  In  localities  swept  by  sea  winds,  salt  derived  from 
wind-whipped  spray  is  usually  a  factor  in  the  floating  dust. 
In  most  of  the  large  seaports  of  the  United  States  the  chlorine 
content  from  this  source  is  made  a  matter  of  systematic  measure- 
ment. The  tendency  of  tools  and  polished  steel  articles  to 
become  rusty  in  the  vicinity  of  the  sea  is  probably  due  as  much 
to  the  chlorine  content  of  the  air  as  to  the  presence  of  excessive 
moisture. 

1  Physics  of  the  Air. 

2  H.  H.  Kimball. 


WIND-BLOWN  DUST  131 

Wind-blown  Dust. — In  regions  of  sparse  vegetation,  where 
the  ground  is  bare,  enormous  amounts  of  loose  rock  waste  are 
moved  hither  and  thither  by  the  wind.  The  increase  of  the 
carrying  power  of  the  wind  with  increment  of  velocity  is  almost 
beyond  belief.  When  the  velocity  of  the  wind  is  doubled  its 
carrying  power  is  increased  sixty-four  fold.  In  regions  of  loose 
rock  waste  the  wind  becomes  a  wonderful  physiographic  factor. 
The  broad,  intermontane  valleys  of  the  plateau  region  have 
been  filled  with  rock  waste,  much  of  which  is  wind-blown; 
and  the  floors  of  the  deeply  filled  valleys  have  been  made  level 
by  wind-blown  dust.  The  plains  to  the  eastward  of  the  Rocky 
Mountains  are  deep  with  wind-blown  dust.  More  dust  and 
rock  waste  is  deposited  in  the  rivers  of  this  region  than  they 
are  able  to  carry.  Platte  River,  popularly  described  as  "a  mile 
wide,  an  inch  deep,  and  bottom  on  top,"  is  an  instance  of  a 
river  drowned  by  the  rock  waste  which  it  cannot  carry. 

Winds  blowing  steadily  for  centuries  have  carried  fine 
rock  waste  from  the  Gobi  far  into  eastern  China,  choking  the 
gorge  of  the  Hoang  in  places  with  wind-blown  dust  more  than 
100  feet  deep.  The  loess  deposits  in  the  lower  course  of  the 
Hoang  are  also  of  wind-blown  dust,  which  has  been  dumped 
into  the  river  in  quantities  greater  than  the  river  could  carry. 
In  1851  the  channel  had  become  clogged  to  the  extent  that  the 
river  broke  its  banks  near  the  city  of  Kaifeng,  abandoned  the 
old  channel  to  the  delta  of  the  Yangste,  and  made  a  new  chan- 
nel to  the  Pechili.  The  sediment  with  which  the  river  had 
clogged  its  channel  was  wind-blown  dust.  In  general,  the 
action  of  the  wind  in  unswarded  regions  is  one  of  leveling.  It 
wears  away  the  high  spots  and  fills  the  low  spots. 

In  regions  of  generous  rainfall,  the  surface  is  covered  with 
vegetation  to  the  extent  that  very  little  rock  waste  is  exposed 
to  the  action  of  the  wind.  About  the  only  physiographic  action 
consists  of  the  formation  of  sand  dunes  to  the  leeward  of  ocean 
and  lake  shores.  In  various  instances  these  have  gradually 
traveled  a  distance  of  several  miles  inland,  ceasing  to  advance 
when  growing  vegetation  has  anchored  the  sand  in  place. 

In  cities  and  much-traveled  rural  districts,  the  wind-blown 
dust  is  picked  up  mainly  from  dirt  streets,  school  playgrounds, 
and  other  unswarded  areas.  The  wind-blown  dust  from  these 
consists  chiefly  of  loose  dirt,  paving  material,  garbage,  finely 


132  THE   DUST   CONTENT   OF   THE  AIR 

pulverized  horse  dung,  and  foliage  dust.  The  dust  carried 
by  winds  blowing  over  areas  of  orchard  and  shrubbery  usually 
contains  the  spores  of  fungi,  pollen  in  season,  the  spores  of 
various  moulds,  the  eggs  of  insects  and  the  dust  scales  of  moths. 
Winds  blowing  over  swampy  areas  are  apt  to  have  a  generous 
content  of  the  micro-organisms  common  to  swamps.  Dry 
air  contains  the  spores  of  micro-organisms;  moist  air  is  often 
rich  in  the  organisms  themselves. 

Bacterium  Content  of  Dust. — Dr.  T.  M.  Prudden  exposed 
Petri  dishes,  each  varnished  with  a  gelatine  culture  medium,  for 
five  minutes  in  different  parts  of  New  York  City.  The  dishes 
were  set  aside  for  several  days.  Each  micro-organism  falling 
on  the  plates  developed  into  a  "colony."  The  colonies  were 
counted  with  the  following  result: 

1.  Ball  ground,  Central  Park,  a  westerly  wind 499 

2.  Union  Square,  at  fountain 214 

3.  A  private  library 34 

4.  An  uptown  dry-goods  store,  near  Broadway 199 

5.  Broadway  and  35th  Street,  small  park 941 

6.  A  cross  street,  after  sweeping 5810 

Examination  of  dust  collected  by  the  author  in  school  rooms 
and  from  the  book  shelves  of  a  public  library  yielded  results 
similar  to  those  obtained  by  Prudden. 

The  foregoing  presents  general  principles  worth  noting: 
micro-organisms  may  fall  to  the  ground  and  become  a  part  of 
the  dust  of  a  public  street.  They  also  float  a  long  time — some 
of  them  permanently — in  the  air.  Exposures  1,5,  and  6  show 
that,  when  the  air  is  in  motion,  the  bacterium  content  is  much 
greater  than  when  the  air  is  still.  Measurements  made  at  the 
direction  of  the  Transvaal  Chamber  of  Mines  showed  that 
dust  particles  I  micromillimeter  in  dimension  required  about 
five  and  one-half  hours  to  fall  a  distance  of  7  feet.  At  the 
Mount  Vernon  laboratory,  particles  of  the  same  dimension 
required  from  six  to  ten  hours  to  fall  9  feet. 

Equally  important  is  the  fact  that,  when  the  air  is  stirred  by 
sweeping,  the  tramp  of  footsteps,  or  the  passage  of  vehicles,  its 
bacterium  content — and  also  its  dust  content — is  much  greater 
than  when  it  is  still. 


CHAPTER  XII 
THE  PRINCIPLES   OF  ATMOSPHERIC  VISIBILITY 

The  transparency  of  the  air  is  a  matter  which  affects  every 
organization  engaged  in  transportation;  the  impairment  of 
visibility  has  led  to  wrecks  without  number  upon  land  and  sea. 
Within  the  past  few  years  wrecks  from  this  cause  have  become 
a  serious  menace  to  air  transportation. 

Standards  for  testing  the  transparency  of  the  air  and  for 
measuring  the  impairment  of  visibility  are  used  here  and  there, 
but  there  is  no  uniformity  among  them;  for  the  greater  part 
they  are  local  as  to  usage.  A  few,  such  as  the  visibility  of  the 
sun's  disk,  the  variability  of  certain  stars,  and  the  sharpness  of 
a  shadow  cast  by  a  rod  on  white  paper,  are  pretty  general  so 
far  as  overhead  observations  are  concerned.1  Seamen  all  over 
the  world  are  pretty  apt  to  judge  visibility  by  the  clearness  of 
the  horizon;  and  the  principle  of  camouflage  is  not  so  much  to 
conceal  a  vessel  as  to  blend  it  with  sea  and  sky  so  that  its  out- 
lines are  indistinguishable.  The  locomotive  engineer  gauges 
visibility  according  to  the  plainness  with  which  he  can  see 
semaphores  during  the  day  and  signal  lights  by  night.  The 
marine  pilot  must  be  able  to  distinguish  the  colors  of  code 
flags  and  smoke  stack  markings,  as  well  as  colored  lights.  The 
air  pilot  must  be  able  to  discern  the  condition  of  the  at- 
mosphere by  the  refraction  of  the  light  passing  through  it  or 
reflected  from  it.  For  almost  all  purposes,  the  problems  of 
visibility  must  be  determined  along  horizontal  lines.  The  im- 
pairment of  visibility  along  vertical  lines  becomes  a  danger 
when  an  airman  cannot  see  his  landing  place. 

Good  visibility  is  safety;   poor  visibility  is  danger. 

Several  factors  are  concerned  in  the  change  from  good  to  poor 

1  Sir  Napier  Shaw  suggests  the  degree  of  visibility  of  the  Zodiacal  Light, 
during  the  season  when  it  is  visible,  as  a  test  of  the  clearness  of  the  air. 
In  many  laboratories  observers  note  the  degree  of  clearness. 

133 


134  PRINCIPLES   OF   ATMOSPHERIC    VISIBILITY 

visibility,  and  these  are  all  contained  within  the  air  itself.    The 
factors  which  impair  visibility  are: 

Fog. 

Thickly  falling  snow. 

Dust  storms  in  arid  regions. 

Very  fine  rain — many  drops  per  cubic  foot. 

Heavy  rain — large  drops. 

Dust  storms  in  swarded  regions. 

A  smoke  pall  held  down  by  a  "lid." 

Moisture  haze  in  a  smoky  air. 

Moisture  haze  or  dust  in  clear  air. 

Refraction  caused  by  the  mixing  of  warm  air  and  cold  air. 

Experience  and  judgment  have  taught  pilots  how  to  avoid 
dangers  that  confront  them.  It  remains  for  meteorologists  to 
fix  definite  standards  and  to  express  varying  visibility  in  terms 
that  are  comprehensible  and  intelligible  to  all.  Moreover, 
in  many  instances,  changes  in  visibility  may  be  forecast  with 
a  high  percentage  of  verification. 

Fog  is  generally  regarded  as  the  chief  factor  in  the  im- 
pairment of  visibility,  but  the  various  other  factors  cannot 
be  arranged  in  an  inflexible  order.  A  desert  simoon  may  impair 
visibility  quite  as  much  as  the  worst  fog;  but  simoons  are 
rarely  in  the  van  of  transportation,  while  fog  is  practically 
coincident  with  every  line  over  which  commerce  is  carried. 
On  the  other  hand,  though  the  blurring  of  outlines  by  refrac- 
tion, and  the  slight  discoloration  of  the  air  by  a  dust  haze  may 
reduce  transparency,  neither  one  is  a  menace  to  safety.  Not 
even  a  moisture  haze  is  disconcerting  unless  it  conceals  the 
horizon. 

In  general,  the  impairment  of  visibility  is  due  to  various 
movements  of  the  air  within  itself.  Except  as  the  wind  picks 
up  fine  dust,  nothing  is  added  to  or  taken  from  the  air  to  make 
the  difference  between  transparency  and  opacity.  The  great 
planetary  movements  of  the  air  need  not  be  considered  here. 
The  cloud  belts  incident  to  them  and  the  conditions  which  pro- 
duce precipitation  are  fairly  understood;  they  are  regular  and 
periodical.  Such  movements  as  are  not  planetary — the  cyclones 
and  the  anticyclones  are  understood  as  to  cause  and  effect,  and 
the  forecasting  of  them  has  become  a  science.  But  there 
are  other  movements,  more  or  less  superficial  and  local,  that 
are  not  so  well  understood.  The  causes  of  them  may  be  known, 


TURBULENCE  135 

but  the  effects  cannot  always  be  forecast.  These  movements 
are  commonly  grouped  under  the  generic  name  of  turbulence, 
and  much  of  the  impairment  of  visibility  is  due  to  them.  Air 
in  convectional  equilibrium  as  to  temperature  and  humidity 
may  be  clear  and  transparent  at  one  time;  a  mixing  process 
may  cause  it  to  be  opaque  a  few  minutes  later.  The  change 
may  be  due  wholly  to  turbulence. 

Turbulence. — Sudden  and  local  movements  of  the  air  are 
due  usually  to  changes  of  temperature.  A  change  of  tem- 
perature produces  a  change  in  pressure;  a  flow  of  air  results, 
and  the  flow — that  is,  the  wind — continues  until  equilibrium  is 
restored.  But  the  moving  of  the  mass  of  air  does  not  always 
produce  a  mixing;  indeed,  the  plane  of  contact  where  it  meets 
another  body  of  air  differing  in  temperature  and  humidity 
sometimes  is  sharply  defined.  The  friction  between  the  two 
masses  frequently  causes  the  condensed  vapor  to  be  rolled 
into  long  windrows  or  billow  clouds.  The  airman  has  learned 
that  visibility  is  impaired  in  this  plane  of  contact  and,  that  in 
passing  from  one  mass  to  another,  he  is  likely  to  experience  a 
sharp  bump. 

As  a  matter  of  fact,  most  of  the  turbulence  which  results 
from  the  mixing  of  air  begins  at  the  ground.  The  "skin  friction" 
of  the  wind  dragging  over  water  reduces  its  velocity  along  the 
plane  of  contact  about  one-third;  over  the  ground  the  reduc- 
tion is  roughly  twice  as  great.1  The  drag  rolls  great  sheets  of 
air  into  volutes.  These,  as  they  are  pushed  upward,  bend  into 
fantastic  shapes,  but  continue  to  rotate  upon  many  axes  of 
many  angles.  Sometimes  a  volute  bends  into  a  ring,  and  the 
ring  itself  rotates  on  a  constantly  changing  diameter  in  ir- 
regular librations.  The  movement,  however,  is  upward  as 
well  as  onward. 

Now,  this  process  of  mixing  is  wholly  different  from  the 
ordinary  convectional  movements  of  the  air.  A  knowledge 
thereof  is  important  to  the  marine  pilot  because  it  is  the  chief 
cause  of  sea  fog  along  the  Atlantic  steamship  lanes.  Thus,  a 
warm,  moist  wind  blows  into  a  region  of  the  drift  of  a  cold 
current.  The  chilling  of  the  water  vapor  quickly  condenses  it 
to  fog,  and  the  churning  movements  of  turbulence  carry  the 
process  of  condensation  higher  and  higher. 

1  G.  I.  Taylor,  Meteorological  Office,  London. 


136  PRINCIPLES    OF    ATMOSPHERIC    VISIBILITY 

To  the  pilot,  the  result  is  not  merely  impairment  of  visi- 
bility; it  may  be  almost  obscurity.  The  airmen  who  crossed 
the  Atlantic  emerged  from  the  sea  fog  with  plane  wings  thickly 
covered  with  ice. 

The  upward  movement  of  turbulence,  "the  railway  of  the 
air"  continues  until  resistance  balances  initial  force — that  is, 
to  an  altitude  which  practically  is  a  lid.  At  this  plane  the  fog 
spreads  out,  forming  stratus  clouds. 

Convectional  Movements. — A  similar  movement  takes  place 
when  air  is  warmed.  An  ascending  movement  occurs  at  the 
focal  area  of  warmth ;  descending  air  flows  in  to  take  its  place. 
To  a  certain  degree,  these  movements  are  planetary;  in  the 
tropics,  ascending  currents  are  the  rule,  and  these  are  balanced 
by  descending  currents  in  higher  latitudes.  Planetary  convec- 
tional  movements  are  pretty  well  known  and  the  limits  of  their 
procession  and  recession  with  the  apparent  motion  of  the  sun  are 
also  known.  The  time,  limits  and  location  of  the  impairment 
of  visibility  resulting  from  these  movements  are  also  pretty  well 
established.  Indeed,  the  Coast  Pilot  Charts  of  the  Hydro- 
graphic  Office  afford  the  information  by  which  the  loci  of  im- 
paired visibility  may  be  determined. 

There  are  other  examples  of  convectional  movements  which 
may  be  regional  but  are  not  planetary.  The  cyclonic  move- 
ments are  ascending  convectional  currents;  the  anticyclones 
are  descending  currents.  The  cyclone  is  very  apt  to  be  an  area 
of  impaired  visibility,  especially  on  the  southeastern  half,  in 
which  rain,  snow  and  fog  may  be  expected.  The  anticyclone 
may  sweep  snow  or  dust  in  blinding  quantities  up  to  a  distance 
of  a  few  hundred  feet  above  ground;  but  it  generally  brings 
the  conditions  of  best  visibility. 

Various  causes  bring  about  local  updraughts  of  small  area. 
Thus,  during  hot  weather,  a  large  area  of  bare  rock  surrounded 
by  greensward  becomes  a  local  radiator  of  heat,  and  a  sharp 
updraught  results.  Vision  through  such  an  updraught  may 
become  blurred,  but  it  is  not  greatly  impaired.  The  airman 
entering  it  gets  a  bump  that  rattles  his  plane,  however.  Descend- 
ing currents  in  the  shape  of  downdraughts  of  small  area  if  rather 
strong  are  apt  to  be  dust-raisers;  they  are  "woollies"  over 
water.  There  may  be  a  slight  blurring  of  outline  due  to  refrac- 
tion, but  there  is  otherwise  but  little  impairment  of  visibility. 


THE  CEILING  OR  LID  137 

The  obscuration  of  outlines  of  objects  at  ground  level  may 
warn  the  airman  in  some  cases  that  he  is  nearing  a  downdraught. 
The  obscuration  may  be  a  dust  storm;  if  under  a  cumulo- 
nimbus cloud  it  is  pretty  certain  to  be  a  dust-storm;  in  snow- 
clad  regions  it  may  be  due  to  wind-blown  snow. 

In  any  case,  although  they  do  not  materially  impair  visi- 
bility, such  local  convectional  movements  are  more  discon- 
certing than  the  cyclones  and  anticyclones.  The  forecasting 
of  these  and  the  charting  of  their  tracks  has  become  a  science. 
Small  local  convectional  movements  cannot  ordinarily  be 
predicted  along  with  general  forecasts;  but  in  various  cases 
they  may  be  predicted  locally.  Thus,  during  abnormally  hot 
days  in  the  valleys  east  of  the  Coast  Ranges  of  California,  the 
updraught  is  so  great  that  strong  sea  winds  prevail  along  the 
coast.  The  air  is  clear  until  after  sunset;  then,  because  of 
rapid  cooling,  fog  billows  roll  in  through  the  Golden  Gate  and 
cover  much  of  the  lowlands. 

The  Ceiling  or  "Lid."  1 — The  paradoxical  epigram,  "air  to 
be  warmed  must  first  be  cooled"  and  vice  versa  is  strictly  true. 
If  a  body  of  air,  having  been  thoroughly  mixed,  comes  to  rest, 
the  temperature  is  not  the  same  throughout  its  mass;  it  is 
cooler  at  the  approximate  rate  of  I  degree  Fahrenheit  for  every 
183  feet  of  ascent  (about  10  degrees  centigrade  per  kilometer). 
It  may  be  defined  as  being  in  convective  equilibrium  while 
in  such  a  condition.  Now,  if  a  body  of  air  at  the  top  be  cooled 
a  few  degrees,  it  contracts  and  becomes  relatively  heavier. 
Because  it  is  heavier,  it  begins  to  drop.  It  is  warmed  by  com- 
pression as  it  descends,  but  it  is  always  surrounded  by  warmer 
air;  so  it  drops  until  it  reaches  a  plane  which  it  cannot  penetrate 
— sometimes  a  layer  of  colder  air;  sometimes  the  ground. 

Similarly,  if  a  mass  of  air  at  the  ground  be  warmed  ever  so 
slightly  above  the  surrounding  air,  immediately  it  begins  to 
ascend,  being  floated  upward  because  it  is  lighter.  It  is  chilled 
by  its  own  expansion  as  it  ascends,  but  as  its  temperature  re- 
mains higher  than  that  of  the  surrounding  air  it  continues  to 
rise  until  it  reaches  a  layer  of  air  as  warm  as  itself.  At  that  level 
it  ceases  to  ascend,  and  spreads  out  instead.  This  plane  there- 
fore becomes  a  "lid."  Perhaps  it  may  reach  the  stratosphere, 

^his  term  was  adopted  by  Sir  Napier  Shaw  in  a  monograph  on  atmos- 
pheric transparency.  It  would  be  difficult  to  coin  a  more  appropriate  name. 


138  PRINCIPLES   OF   ATMOSPHERIC    VISIBILITY 

for  the  stratosphere  is  a  planetary  lid  that  envelopes  the  con- 
vectional  air;  but  a  lid  may  form  anywhere  between  the  ground 
and  the  stratosphere.  Wherever  a  layer  of  warm  air  rests  on 
one  of  still,  cold  air  a  lid  is  formed.  Smoke,  dust,  and  other 
fine  foreign  matter  spread  out  to  form  stratus  cloud  when  it 
reaches  such  a  lid.  If  the  two  air  layers  are  not  turbulent 
there  is  little  or  no  mixing. 

Low  stratus  clouds  indicate  the  height  of  a  lid  near  the 
ground — half  a  mile  or  more;  but  the  stratiform  appearance 
is  seen  to  best  advantage  when  the  clouds  are  not  higher  than 
30  or  40  degrees  above  the  horizon.  Near  the  zenith  they  lose 
their  stratiform  shape,  being  then  seen  in  "elevation"  and  not 
in  "plan";  but  frequently  they  indicate  themselves  to  the 
practised  eye  of  an  observer.  The  strato-cumulus  clouds  that 
follow  an  anticyclone  also  indicate  a  lid.  The  high  fog  that 
completely  covers  the  sky  at  heights  varying  from  7000  to 
10,000  feet — practically  a  stratus  or  an  alto-stratus  cloud — ;is 
a  lid.  Cross-winds  at  a  very  considerable  height  likewise  may 
indicate  a  lid. 

The  presence  of  a  lid  has  much  to  do  with  the  comfort  of 
the  airman.  In  penetrating  a  lid  the  plane  is  apt  to  get  a  sharp 
bump.  If  lightly  ballasted,  a  free  balloon  may  rebound  after 
descending  upon  ajid  and  shoot  upward  several  hundred  feet. 
Sir  Napier  Shaw  has  called  the  attention  of  aeronauts  to  this 
possibility. 

A  low  lid  affects  visibility  to  a  marked  degree.  Under  the 
lid,  fine  floating  dust,  smoke  and  the  various  gases  of  com- 
bustion spread  out  in  stratus  cloud  and  greatly  impair  visibility. 
The  famous  London  fog  is  due  to  the  persistence  of  a  low- 
lying  lid. 

The  impairment  of  visibility  depends  partly  on  the  height  of 
the  lid  and  partly  on  the  character  of  the  content  of  the  air 
underneath  it.  In  open,  swarded  regions  where  the  air  is  free 
from  pollution,  not  much  impairment  of  visibility  is  likely  to 
exist.  In  regions  where  soft  coal  is  used  as  power  fuel,  chimney 
products  may  accumulate  to  such  an  extent  that  impairment 
becomes  a  very  serious  matter;  and  the  lower  the  lid,  the 
greater  becomes  the  impairment.  Obscurity  is  apt  to  grow 
until  increasing  pressure  breaks  the  lid  and  brings  about  a.  clear- 
ing of  the  air. 


FOG,  CLOUD,  MIST  139 

Pressure  is  an  important  factor  in  visibility.  When  the  air 
is  misty  and  the  seeing  generally  is  poor  a  very  slight  increase 
in  pressure  clears  it  up  at  once.  As  a  cyclonic  depression  ad- 
advances  the  seeing  becomes  poorer,  because  of  rain  or  snow, 
until  the  trough  passes.  Then  the  seeing  at  once  begins  to 
improve,  with  increase  of  pressure. 

The  foregoing,  turbulence,  convection,  and  inversion — that 
is,  the  formation  of  a  lid — are  the  principal  movements  of  the 
air  which  impair  visibility.  The  factors  themselves  are  moisture, 
dust,  smoke  and  refraction  of  light.  The  dust  and  smoke  differ 
merely  in  origin;  the  moisture  may  appear  in  the  form  of  fog, 
mist,  rain,  or  snow. 

Fog,  Cloud,  Mist. — In  marine  transportation,  fog  is  the  worst 
factor  in  the  impairment  of  visibility.  Practically  it  is  the  only 
one.  If  the  temperature  is  brought  below  the  dew-point,  fog 
results  from  condensation  of  the  water  vapor.  The  brisker  the 
wind,  the  thicker  the  fog  blanket.  A  convectional  updraught 
does  not  destroy  the  turbulence;  convection  merely  carries  it 
higher. 

The  thickening  of  a  sea  fog  is  an  illustration.  If  the  sea 
water  is  colder  than  the  air,  which  is  the  case  when  polar  waters 
intrude  within  lower  latitudes,  warm  air  blowing  over  it  will 
give  up  its  vapor  in  the  form  of  fog.  So  long  as  the  eddying 
movements  of  the  air  are  constantly  bringing  warm  air  next 
the  cold  water,  fog  condensation  increases.  The  fog  blanket 
thickens;  its  upper  part  marks  the  height  at  which  the  tem- 
perature of  the  air  is  above  that  of  saturation. 

The  fogs  of  the  Newfoundland  Banks  have  been  the  terror 
of  the  sailing  route  between  American  and  British  ports.  They 
will  prove  a  much  greater  hazard  to  air  transportation  unless  a 
circuitous  southerly  route  is  followed.  An  Arctic  current  and 
a  southwest  wind  laden  with  water  vapor  constitute  the  work- 
ing machinery  of  this  fog  factory.  Radio-telegraphy  now  fore- 
warns the  sailing  master  when  and  where  he  will  encounter  the 
fog  blanket  that  hovers  over  an  ocean  graveyard.  Times  and 
positions  of  these  fogs  are  approximately  known,  but  definite 
forecasts  cannot  now  be  made. 

During  summer,  fog  along  the  steamship  lanes  in  the  vicinity 
of  the  Banks  averages  between  ten  and  twenty  per  cent  of  the 
time;  in  winter  it  may  be  expected  about  one-third  of  the  time. 


140  PRINCIPLES    OF    ATMOSPHERIC    VISIBILITY 

If  an  iceberg  is  sighted  dead  ahead  during  foggy  weather  it  is 
usually  so  near  that  avoidance  is  difficult.  A  steamship  can 
stop  or  it  may  back.  An  air  plane  can  do  neither. 

From  time  to  time  experiments  in  oiling  the  area  of  fog- 
covered  waters  have  been  tried;  but  the  oil  film  has  no  effect 
on  the  fog.  The  fog  comes  from  the  air  and  not  from  the  water 
and  the  oil  film  does  not  warm  either  one. 

The  fogs  of  the  Pacific  Coast  occur  usually  in  early  evening 
and  may  continue  after  sunrise.  In  southern  California  the 
coast  fog  may  be  high  above  ground.  It  is  then  the  "velo,"  or 
veil.  The  fogs  of  the  middle  Atlantic  Coast  are  usually  as- 
sociated with  cyclonic  movements.  To  a  certain  extent  they 
are  of  the  nature  of  city  fogs,  being  encouraged  by  the  smoke 
and  dust  incident  to  city  industries.  In  the  larger  harbors  fog 
may  be  forecast,  but  not  with  a  high  degree  of  verification. 
Even  a  light  fog  ties  up  shipping  pretty  effectually. 

A  light  fog  may  be  more  opaque  than  a  heavy  rainfall. 
Direct  rays  of  light  do  not  penetrate  a  dense  fog  more  than  a 
few  rods.  The  light  is  scattered  by  reflection.  The  amount 
of  water  contained  in  a  cubic  foot  of  saturated  air  at  67°  is 
6.2  grains.  If  the  temperature  be  reduced  to  42°,  approximately 
one-half  the  vapor,  3.1  grains,  will  appear  as  fog,  and  this 
amount  is  sufficient  to  produce  a  very  dense  fog. 

In  practise,  a  single  rule  must  guide  a  pilot;  when  the  limit 
of  visibility  is  less  than  the  distance  required  to  make  a  stop, 
there  is  danger.  In  traversing  sea  fogs,  where  other  vessels 
are  not  likely  to  be  encountered,  sailing  masters  have  expressed 
the  opinion  that  quite  as  much  danger  exists  at  half  speed  as 
at  full  speed.  Perhaps  this  is  true  if  one  considers  a  collision. 
Nevertheless,  at  full  speed  an  average  of  twice  as  many  chances 
of  collision  will  occur,  for  the  vessel  will  meet  an  average  of 
twice  as  many  other  vessels  in  a  given  time. 

To  the  airman  there  is  no  difference  between  fog  and  cloud, 
so  far  as  the  impairment  of  visibility  is  concerned.  For  the 
greater  part,  a  pilot  may  fly  above  stratus  clouds  if  the  air 
is  not  clear  below  them;  and  airmen  usually  can  find  plenty  of 
room  under  the  alto-stratus  clouds  of  an  overcast  sky.  But  an 
airman  who  has  once  encountered  a  cumulo-nimbus  cloud,  or 
even  large  masses  of  low  cumulus  clouds,  is  not  apt  to  repeat 
the  experience.  In  many  cases  both  fog  and*  cloud  may  be 


,   VISIBILITY  AFFECTED  BY  RAIN  OR  SNOW  141 

avoided;  but  one  cannot  avoid  a  fog  when  it  shrouds  a  landing 
place.  And  while  fog  turbulence  is  slight,  cloud  turbulence 
may  be  very  great,  and  this  is  notably  the  case  with  cumulus, 
and  cumulo-nimbus  clouds. 

It  is  not  easy  to  draw  the  line  between  fog  and  mist.  In- 
asmuch as  the  droplets  of  mist  are  much  larger  than  those  of 
fog,  they  do  not  scatter  so  much  light;  moreover,  measured 
per  cubic  unit  of  air,  there  are  not  so  many  of  them.  At  times 
one  may  see  a  gray  moisture  haze  in  the  direction  of  the  horizon. 
A  little  experience  enables  one  to  distinguish  it  from  a  dust 
haze.  It  is  never  thick  enough  to  impair  seeing  materially. 

Humid  air  is  not  quite  so  clear  as  dry  air,  but  it  rarely  loses 
transparency  to  the  extent  that  it  impedes  transportation. 
Nevertheless,  the  impairment  of  visibility  by  air  that  was  merely 
very  moist  has  been  the  critical  point  of  several  suits  in  which 
railways  were  involved. 

Rain  and  Snow. — It  is  not  often  that  rain,  per  se,  falls  so 
fast  that  the  seeing  is  badly  impaired ;  but  now  and  then  this 
happens.  Very  heavy  downpours  may  limit  the  vision  to  less 
than  a  few  rods.  But  downpours  of  this  sort  are  not  common, 
and  if  the  seeing  is  passable  for  1000  feet  ahead,  danger  is 
largely  avoidable. 

Some  of  the  light  passing  through  raindrops  is  refracted; 
some  is  reflected  and  otherwise  scattered.  The  outlines  of  an 
object  which  normally  is  distinct  may  be  obliterated,  but  its 
mass  is  likely  to  be  seen.  In  this  respect  rain  differs  from  fog. 
Even  if  the  combined  surface  of  the  drops  next  the  observer 
is  sufficient  to  form  a  screen,  the  screen  is  partly  transparent, 
but  a  fog  screen  is  practically  opaque.  If  a  rain-drop  be 
broken  into  water  particles  of  fog  size,  their  aggregate  surface 
is  several  million  times  that  of  the  rain-drop.  The  screening 
power  of  the  fog,  therefore,  is  vastly  greater  than  that  of  the 
rain-drop  and  so  also  is  the  amount  of  light  scattered. 

A  fast-falling  snow  is  about  as  bad  for  visibility  as  an  ordi- 
nary fog.  If  blizzard  conditions  prevail,  the  snow  may  be 
broken  into  a  fine  ice  dust  quite  as  opaque  as  a  thick  fog.  The 
airman  may  avoid  a  snow  squall  of  small  area  by  flying  around 
it;  the  locomotive  engineer  and  the  marine  pilot  must  push 
through  it.  The  danger  point  is  reached  when  a  snowfall  hides 
semaphores  or  obscures  signal  lights. 


142  PRINCIPLES    OF    ATMOSPHERIC    VISIBILITY 

Dust  Storms. — In  arid  regions  winds  of  the  simoon  type 
are  not  uncommon.  Frequently  they  carry  heavy  clouds  of 
dust  far  beyond  desert  boundaries  into  fertile  regions.  The 
Santa  Ana  of  southern  California  is  an  example.  Dust  storms 
originating  in  the  plains  states  sometimes  carry  their  content 
as  far  east  as  the  Mississippi.  So  far  as  the  impairment  of 
seeing  is  concerned  their  effect  does  not  reach  more  than  a 
few  hundred  feet  above  ground.  The  haze  of  fine  and  highly 
electrified  dust  which  commonly  hangs  in  the  air  after  a  dust 
storm  is  not  very  opaque,  but  it  extends  much  higher  above 
ground.  It  may  persist  for  several  days.  The  airman  may 
fly  above  a  dust  storm,  but  if  it  blankets  a  landing  place  it 
becomes  a  positive  danger. 

The  ordinary  dust  haze,  a  bluish  tinge  observable  against  a 
dark  background,  does  not  impair  seeing.  Frequently  it  has 
the  density  which  the  landscape  artist  terms  "atmospheric 
effect."  The  sea  haze,  on  the  other  hand,  may  be  disconcerting 
because  it  may  hide  the  distinctive  marks  of  nearby  vessels. 
The  sea  haze  has  been  an  interesting  factor  in  naval  strategy 
because  of  this  fact.  Frequently  it  is  dense  enough  to  inter- 
fere with  signaling,  even  with  helio-apparatus.  Calmness  of 
the  air  is  a  condition  necessary  to  the  formation  of  the  blue 
dust  haze  and  the  sea  haze. 

The  Smoke  Pall. — The  smoke  that  hovers  over  manufac- 
turing districts  differs  materially  from  that  of  forest  fires,  being 
composed  largely  of  free  carbon  and  hydrocarbons.  In  moist 
weather  water  drops  varnished  with  tarry  matter  are  mixed 
with  smoke  carbon.  Mixed  with  stack  products  are  sulphur 
gases — sulphur  trioxide  and  sulphur  dioxide.  These  are  chem- 
ically active  and  in  the  presence  of  moisture  become  very 
effective  nuclei  of  condensation. 

In  manufacturing  districts  where  soft  coal  is  extensively 
used  as  power  fuel  the  smoke  pall  may  be  dense  enough  to 
hide  the  outlines  of  large  objects  at  a  distance  of  4  or  5  miles. 
A  low-lying  lid  holds  the  smoke  pollution  close  to  the  ground 
and  impairs  seeing  very  materially.  The  airman  may  fly  above 
the  lid  into  a  region  of  clear  air;  the  marine  pilot,  except  in 
harbors,  is  out  of  the  way  of  smoke;  the  locomotive  engineer, 
who  cannot  avoid  the  smoke  pall,  sometimes  finds  it  discon- 
certing. It  rarely  interferes  with  signaling. 


VISIBILITY  AND  REFRACTION  OF  LIGHT  143 

Under  ordinary  circumstances  the  diffusion  of  smoke  is 
so  rapid  that  it  is  rarely  visible  at  a  distance  of  more  than  40 
or  50  miles  from  the  source  of  pollution.  At  this  distance  a 
dirty-appearing  horizon  is  about  the  extent  of  the  impairment 
of  seeing.  From  Chicago  to  South  Bethlehem  the  region  is  one 
of  almost  continuous  manufacture;  nevertheless,  the  combined 
smoke  pollution  of  the  wide  region  is  rarely  discernible  at  the 
Atlantic  Coast. 

Refraction  of  Light. — Rays  of  light  passing  through  bodies 
of  air  differing  in  density  are  bent  from  their  original  direction. 
The  outlines  of  objects  therefore  reach  the  observer  more  or  less 
distorted.  The  blurring  of  outlines  one  notices  along  a  rail- 
way track  is  an  example.  At  a  distance  of  half  a  mile  an  ap- 
proaching locomotive  appears  as  a  dark  mass  without  outline. 
The  imperfect  mixture  of  warm  air  and  cold  air  causes  the 
scattering  of  light.  A  boss  of  rock  projecting  from  the  coast, 
or  surrounded  by  greensward,  produces  a  similar  effect  notice- 
able to  the  air  pilot. 

Refraction  of  this  sort  is  a  menace  to  safety  whenever  it 
conceals  the  outlines  of  objects  which  should  be  recognizable 
beyond  stopping  distance.  A  locomotive  engineer  who  loses 
time  in  order  to  make  certain  of  semaphore  signals,  and  is 
censured  therefor,  is  in  about  as  bad  a  position  as  one  who  is 
disciplined  for  running  past  them  for  the  same  reason.  Usually 
the  judgment  that  conies  with  experience  enables  the  engineer 
to  observe  the  necessary  precautions. 

The  mirage  of  arid  regions,  especially  of  the  desert,  is  dis- 
concerting at  times.  It  hides  landmarks  which  are  necessary 
to  the  safety  of  the  traveler;  along  the  railways  of  arid  regions 
it  may  disconcert  trainmen.  When  it  is  below  the  eye  of  the 
observer  it  has  the  appearance  of  a  distant  body  of  water  which 
reflects  the  sky.1  It  is  observable  only  when  the  eye  is  not 
more  than  4  or  5  degrees  above  the  level  of  the  apparent  surface. 
The  angle  is  so  critical  that  a  change  of  level  of  2  feet  on  the 

1  Trained  observers  in  arid  regions  are  of  the  opinion  that  the  ordinary 
desert  mirage  is  due  to  the  reflection  of  light  from  the  plane  of  contact  of  two 
layers  of  air  resting  one  upon  the  other.  The  experience  of  the  author, 
covering  many  years  in  the  desert  region  of  western  North  America,  favors 
this  explanation.  In  his  "Light,"  Professor  Hastings  explains  it  as  a  case 
of  refraction,  and  this  view  is  held  also  by  Humphreys. 


144  PRINCIPLES   OF   ATMOSPHERIC    VISIBILITY 

part  of  the  observer  may  destroy  the  illusion.  Such  a  mirage 
may  be  apparent  to  a  man,  and  not  to  a  child  standing  beside 
him. 

The  desert  mirage  is  disconcerting  as  well  as  deceiving. 
Surveyors  occasionally  are  compelled  to  suspend  work,  and 
locomotive  engineers  are  sometimes  deceived  concerning  the 
locations  of  sidings  and  signals.  A  cattleman  who  unwisely 
attempted  to  drive  a  herd  of  several  thousand  cattle  across  the 
Colorado  Desert  lost  the  entire  herd.  The  cattle,  becoming 
thirsty,  grew  very  nervous.  The  mirage  deceived  them  and 
they  stampeded  to  their  death.  During  a  battle  between 
British  troops  and  Turks  in  the  arid  plains  of  the  Tigris,  a 
desert  mirage  concealed  the  Turks  so  effectually  that  fighting 
was  temporarily  suspended. 

From  the  nature  of  the  case,  forecasts  of  the  impairment  of 
visibility  due  to  refraction  are  out  of  the  question.  However, 
it 'may  be  safely  assumed  that  when  a  light  wind  is  blowing 
there  will  be  no  trouble  from  this  source.  In  desert  regions  the 
whirls,  sometimes  known  as  "sand  spouts,"  indicate  the  absence 
of  surface  winds.  They  indicate  the  breaking  up  of  a  lid,  but 
their  effect  on  visibility  is  slight. 

Forecasting  Conditions  of  Visibility. — Some  of  the  funda- 
mental conditions  of  visibility  have  been  discussed  in  detail. 
Fogs  cannot  be  forecast  with  any  degree  of  certainty,  but  local 
conditions  may  indicate  their  probability,  especially  in  the 
case  of  the  coast  fogs  already  noted.  Along  the  steamship  lanes 
between  American  and  British  ports,  time  and  place  are  indi- 
cated, not  by  forecast  but  by  probability.  City  fogs,  which 
are  due  largely  to  pollution,  cannot  be  forecast.  They  are  indi- 
cated when  the  humidity  is  high  and  the  smoke  pollution  great. 
They  disappear  with  a  slight  rise  of  temperature.  It  is  well  to 
bear  in  mind  that,  with  temperature  close  to  the  dew-point,  a 
fall  of  a  very  few  degrees  may  fill  the  air  with  a  dense  fog ;  a  rise 
in  temperature  ever  so  slight  may  change  foggy  air  to  clear  air. 
Impairment  of  visibility  due  to  smoke  pollution  cannot  be 
forecast.  When  due  to  a  lid,  a  rise  of  barometric  pressure  indi- 
cates a  clearing  of  the  air,  which  may  take  place  in  an  hour. 
In  other  words,  a  lid  indicates  suspended  convection. 

The  best  seeing  comes  with  an  anticyclone,  the  forecasting 
of  which  is  pretty  certain  to  be  verified.  Other  highs  indicate 


FORECASTS  OF  VISIBILITY  145 

good  seeing  if  the  air  is  dry,  and  dryness  of  the  air,  when  not 
polluted,  is  fundamental  to  its  clearness.  Rain  or  snow,  and 
mist  are  pretty  certain  to  accompany  a  cyclone.  A  cirrus 
haze  at  the  eastern  horizon  and  a  white  sky  overhead  are  fol- 
lowed by  gathering  clouds  which  increase  in  thickness,  and  by 
precipitation.  These  changes  can  be  forecast,  both  as  to  time 
and  place,  with  a  fair  degree  of  certainty.  The  advancing  half 
of  a  cyclonic  depression  is  an  area  of  increasing  impairment  of 
seeing;  the  receding  half  is  one  of  improving  visibility.  The 
same  is  true  of  the  V-shaped  depressions  of  western  coasts  in 
high  latitudes.  On  the  front  of  the  V  the  seeing  grows  worse; 
at  the  rear,  it  constantly  improves.  In  each  case  the  decreasing 
pressure  brings  foul  vision;  the  increasing  pressure,  good  seeing. 
Stagnation  of  the  air  almost  always  brings  haziness,  but 
farely  to  an  extent  that  interferes  with  good  seeing.  In  some 
cases,  such  as  the  "stranded  Bermuda  high"  it  may  be  roughly 
forecast.  The  haziness  resulting  from  stagnation  may  inter- 
fere with  the  long-distance  helio-signals  occasionally  necessary, 
or  with  the  long-distance  sighting  in  geodetic  surveys;  other- 
wise, the  impairment  is  not  of  consequence. 


CHAPTER  XIII 
THE  DAILY  WEATHER  MAP:   STORMS 

THE  DAILY  WEATHER  MAP 

The  daily  weather  map  is  a  bird's-eye  view  of  the  United 
States  with  respect  to  temperature,  pressure,  wind,  and  storm 
at  8  o'clock  in  the  morning,  seventy-fifth  meridian,  standard 
time.  A  few  minutes  before  8  o'clock  more  than  two  hundred 
observers  are  busy  recording  all  weather  conditions  covering  the 
various  stations.  These  observations  are  completed  by  8 
o'clock,  morning  and  evening,  and  are  promptly  telegraphed 
to  the  Weather  Bureau  at  Washington  in  coded  form.  So 
carefully  and  thoroughly  is  the  work  done  that  a  few  code 
words  from  each  station  contain  all  the  necessary  information 
concerning  temperature,  pressure,  direction  of  the  wind,  rain 
or  snow,  cloudiness,  moisture,  thunder-storms,  fog,  and  other 
phenomena. 

Making  the  Daily  Weather  Map. — At  the  central  office, 
and  also  at  certain  other  designated  offices,  the  figures  and 
other  information  are  charted  on  a  base  map  containing  the  name 
and  position  of  each  station,  the  boundaries  of  the  states,  and 
the  outline  of  the  United  States.  For  the  sake  of  clearness, 
all  other  features  and  names  are  omitted.  Blue  lines  are  drawn 
to  indicate  isotherms;  red  lines  similarly  indicate  isobars.  When 
the  isobars  are  completed  it  will  be  found  that  some  of  them  are 
roughly  concentric,  inclosing  irregularly  shaped  ellipses.  In 
some  of  these  the  pressure  is  highest  at  the  center;  in  others, 
the  center  is  the  point  of  lowest  pressure.  These  are  the  highs 
and  the  lows  that  indicate  storm  centers — that  is,  anticyclones 
and  cyclones. 

In  order  to  give  the  forecaster  additional  information,  the 
direction  of  the  wind  and  the  sky  condition  must  be  noted. 
These  are  shown  in  each  case  by  a  circle  pierced  with  an  arrow. 

146 


DISTRIBUTING  WEATHER  INFORMATION  147 

The  arrow  points  the  direction  of  the  wind.  If  the  sky  is  clear, 
the  circle  of  the  arrow  is  left  clear;  if  partly  cloudy,  half  the 
circle  is  blackened;  if  cloudy,  all  the  circle  is  blackened.  An 
R  in  the  circle  indicates  rain ;  5"  means  snow ;  and  M  means 
that  the  report  for  that  particular  station  is  missing.  All  lo- 
calities in  which  rain  or  snow  is  falling  are  shaded.  The  map 
thus  finished  is  the  daily  weather  map,  and  from  it  the  forecasts 
of  the  following  twenty-four  to  thirty-six  hours  are  made. 
These  also  may  include  information  discovered  by  the  long- 
distance forecasts. 

Distributing  Weather  Information. — The  base  maps  on 
which  the  information  is  to  appear  are  distributed  to  such  sta- 
tions as  issue  daily  weather  maps.  As  soon  as  the  matter  de- 
scribed in  the  preceding  paragraph  has  been  placed  graphically 
on  the  map,  it  is  reproduced  by  a  quick  process  in  the  form  of 
a  printing  plate.  The  matter  for  the  plate  is  usually  ready  by 
half-past  nine  o'clock,  and  is  printed  on  the  base  maps  which 
are  usually  folded,  wrapped,  and  addressed  within  a  short  time. 
At  the  New  York  City  Station  about  3000  maps  are  required 
for  the  daily  issue.  They  are  sent  to  shippers,  railroad  offices, 
merchants,  newspapers,  educational  institutions,  post  offices, 
and  public  places  of  various  sorts.  Almost  every  daily  paper 
publishes  a  resume  of  the  weather  map;  some  reproduce 
the  map  itself.  All  told,  the  daily  forecast  is  so  widely  pub- 
lished that  it  is  within  almost  instant  reach  of  everyone  within 
the  main  body  of  the  country. 

Features  of  the  Weather  Map. — The  chief  desire  of  the 
public  is  to  learn  whether  the  weather  during  the  succeeding 
few  hours  is  likely  to  be  pleasant  or  stormy,  warmer  or  colder, 
clear  or  cloudy,  quiet  or  windy.  These  are  features  that  affect 
all  people;  and  the  daily  weather  map  answers  the  questions 
correctly  a  little  more  than  four  times  in  five.  The  verification 
of  rain  or  of  snow  practically  is  four  times  in  five;  of  tempera- 
ture and  wind  direction,  rather  better  than  four  times  in  five.1 

A  study  of  the  weather  map  will  show  the  area  or  areas  in 
which  rain  or  snow  was  falling  at  the  time  of  observation ;  it  will 
show  where  freezing  temperatures  may,  or  may  not  have  ex- 

1  The  percentage  of  verification  varies  with  locality.  In  California, 
where  the  rain  and  temperature  conditions  are  seasonal,  the  percentage  of 
verification  is  high. 


148 


THE   DAILY  WEATHER   MAP:    STORMS 


MOVEMENT  OF  WEATHER  CONDITIONS  149 

isted;  it  will  show  the  areas  of  clear,  cloudy  and  windy  condi- 
tions. The  man  in  Chicago  may  learn  at  a  glance  the  weather 
conditions  at  Los  Angeles,  New  York  City,  Winnipeg,  Key 
West,  Bermuda,  or  Havana.  A  merchant  who  has  shipped 
perishable  goods  may  learn  whether  or  not  his  consignments 
are  threatened  by  washouts,  snow  blockades,  or  cold  waves. 
In  other  words,  the  daily  weather  map  is  very  much  more  than 
a  mere  bird's-eye  view  of  the  air  and  its  conditions;  it  is  a  sur- 
vey— a  topographic  map — with  measured  values. 

STORMS 

The  Movement  of  Weather  Conditions. — The  ripples,  whirl- 
pools, and  waves  of  a  river  are  carried  along  in  its  flow;  so  also 
the  waves  and  whirlpools  of  the  air  are  carried  along  with  the 
great  streams  of  the  air.  Throughout  the  greater  part  of  the 
United  States,  this  movement  is  from  a  westerly  to  an  easterly 
quadrant — that  is,  the  greater  part  of  the  main  body  is  in  the 
belt  of  Prevailing  Westerlies.  The-  Gulf  Coast,  together  with 
the  Florida  Peninsula  are  in  the  Trade  Wind  belt  in  summer, 
but  not  in  winter.  Therefore,  such  movements  as  cyclonic 
areas  or  lows  will  move  from  a  westerly  to  an  easterly  quadrant 
with  the  velocity  of  the  general  movement  of  the  air. 

The  paths  of  the  principal  types  of  cyclonic  storms  which 
have  already  been  described  are  shown  on  the  accompanying 
map.  They  are  discovered  by  means  of  the  isobars.  That  is, 
somewhere  in  the  middle  western  part  of  the  United  States 
an  isobar  of  30.00  inches  will  be  found  to  inclose  an  elliptical 
area.  Within  this  area  isobars  are  drawn  for  every  tenth  of  an 
inch  of  decreasing  pressure.  This  area  is  a  low,  and  probably 
an  area  of  updraught;  if  the  pressure  is  below  29.50  inches 
it  is  pretty  certain  to  be  a  strong  updraught,  and  the  arrows 
which  indicate  wind  direction  are  pointing  toward  the  center 
of  the  low. 

If  the  pressure  at  the  central  part  of  the  low  is  only  two-  or 
three-tenths  below  30.00  inches,  the  updraughft  is  not  very  strong 
and  the  winds  blowing  into  the  low  are  light.  Rain  or  snow 
may  or  may  not  be  falling.  On  the  other  hand,  if  the  pressure 
within  the  low  is  29.50  inches  or  less,  rain  or  snow,  followed  by 
heavy  winds,  is  pretty  certain  to  occur,  mainly  on  the  east  and 


150       THE  DAILY  WEATHER  MAP:  STORMS 

south  sides  of  the  cyclones;  if  the  pressure  falls  below  29.00 
inches  a  violent  storm,  with  winds  from  whole  gale  to  storm 
strength,  is  certain. 

The  various  types  of  cyclonic  storms  differ  but  little  in 
character,  and  their  names  apply  to  the  locality  where  they 
originate  or  are  first  observed.  Thus,  they  are  variously  known 
as  "Alberta,"  "North  Pacific,"  "Northern  Rocky  Mountain," 
"Colorado,"  "Texas,"  "Central,"  and  "West  Indian."  Other 
names  occasionally  are  used  in  designating  the  storms.  The 


Red-way's  Physical  Geography. 
Wind,  cloud,  and  precipitation  in  a  cyclonic  storm. 

system  employed  by  the  Weather  Bureau  is  one  of  convenience 
rather  than  of  scientific  value.  About  a  third  of  the  storms 
that  cross  the  continent  are  of  the  Alberta  and  North  Pacific 
type. 

The  map,  p.  148,  shows  the  isobars  of  a  cyclonic  storm. 
The  low  pressure  at  the  center  indicates  a  storm  of  unusual 
intensity;  this  is  indicated  also  by  the  closeness  of  the  isobars. 
In  other  words,  the  pressure  gradient  is  steep,  when  the  isobars 
are  close,  and  this  also  indicates  the  degree  of  violence  of  the 
storm. 


STORM  MOVEMENTS  151 

Observations  covering  more  than  twenty  years  show  that 
winter  storms  of  the  United  States  advance  at  the  rate  of  a 
little  more  than  700  miles  per  twenty-four  hours;  summer 
storms  cover  about  500  miles.  These  figures  differ  from  the 
values  obtained  by  the  British  Meteorological  Office,  576  miles 
and  474  miles  per  day  respectively.  The  progress  of  the  cyclone 
is  merely  the  velocity  of  the  general  drift  of  air,  and  this  varies 
in  different  latitudes,  and  at  different  times. 

Inasmuch  as  the  storm  tracks  of  the  different  types  are  fairly 
regular  in  position,  and  the  velocity  of  progress  is  known,  it  is 
not  difficult  to  forecast  the  position  of  a  storm  from  day  to  day; 
that  is,  a  storm  center  which  is  over  Cincinnati  may  be  ex- 
pected to  reach  Philadelphia  or  New  York  at  about  the  same 
hour  on  the  following  day.  Fast  express  trains  run  at  a  rate  of 
speed  that  rarely  varies;  the  cyclonic  storm  moves  also  at  a 
fairly  uniform  speed.  The  express  train  does  not  ordinarily 
leave  its  steel-bound  track;  in  this  respect  it  differs  from  the 
cyclonic  storm  which  occasionally  does  swerve  from  its  ex- 
pected track  to  the  confounding  of  the  forecaster.  This  is 
likely  to  happen  about  once  in  five  times. 

Let  us  suppose  that  a  storm  of  the  Alberta  type,  after  reach- 
ing the  Great  Lakes,  takes  a  dip  southward  and  passes  off  the 
coast  somewhere  near  Cape  May,  instead  of  following  a  pre- 
dicted course  across  New  York.  In  the  eastern  part  of  the 
United  States  practically  all  forecasts  north  of  Cape  Hatteras 
will  be  upset.  Instead  of  rain,  central  New  York  and  Massa- 
chusetts will  have  clear  or  partly  cloudy  weather.  Baltimore  and 
Washington  will  have  cloudiness,  easterly  winds  and  rain, 
instead  of  clear  or  partly  cloudy  skies. 

Not  only  may  a  cyclonic  storm  swerve  from  its  predicted 
track;  it  also  may  fail  to  produce  the  rain  or  the  snow  which, 
according  to  popular  tradition,  constitutes  the  storm.  As  a 
matter  of  fact,  the  rain  and  the  snow  are  merely  an  incident 
in  a  cyclonic  movement.  The  essential  feature  of  cyclone 
mechanics  is  the  updraught.  Now,  in  its  progress  if  the  cyclone 
invades  an  area  of  very  dry  air,  the  updraught  may  not  be 
cooled  to  the  temperature  of  condensation;  in  such  a  case 
there  will  be  no  precipitation.  All  lows  are  not  rain  storms  or 
snow  storms  in  the  ordinary  meaning;  but  practically  all  the 
rain  and  snow  that  fall  on  large  areas  accompany  winter  lows. 


152  THE   DAILY  WEATHER   MAP:    STORMS 

Let  us  take  a  low  which  is  central  in  Illinois.  The  wind  is 
blowing  into  it  from  all  quadrants,  to  fill  the  updraught.  The 
storm  is  preceded  by  a  wind  from  an  easterly  quadrant  and 
clears  with  one  from  a  westerly  quadrant,  which  is  apt  to  settle 
in  the  northwest.  Within  the  storm  area  the  winds  acquire  a 
spiral  motion,  whirling  upward  contra-clockwise  as  they  ap- 
proach the  updraught.  The  whirl  brings  warm  and  moist  air 
from  a  southerly  region  to  the  easterly  side  of  the  low,  where 
the  air  is  colder  and  the  temperature  nearer  to  the  dew-point— 
that  is,  to  condensation.  For  this  reason,  most  of  the  precipi- 
tation is  on  the  east  and  south  sides  of  the  low.  On  the  west 
side  colder  and  drier  air  is  blowing  from  the  west  and  the  north- 
west and,  being  colder,  is  drawn  into  the  updraught  to  a  less 
extent  or  perhaps,  not  at  all.  Westerly  and  northwesterly  winds, 
therefore,  usually  are  clearing  winds. 

Just  as  the  trough  of  a  wave  is  followed  by  a  crest,  so  a  low 
is  pretty  apt  to  be  followed  by  a  high;  and  cyclonic  storms  of 
the  Alberta  type  are  frequently  followed  by  crests  or  waves  of 
cold  air  from  high  latitudes.  If  a  winter  high  pressure  area  lies 
over  the  northwestern  part  of  North  America,  and  a  low  forms 
anywhere  in  the  vicinity  of  this  area,  a  flow  from  the  high 
to  the  low  will  naturally  follow.  This  means  that,  in  order  to 
fill  the  low,  the  clearing  northwest  winds  must  also  be  descending 
currents;  and,  as  a  matter  of  fact,  they  flow  along  the  surface, 
lifting  the  warm  air  above  them.  In  their  flow  into  lower  lati- 
tudes and  their  descent,  they,  too,  acquire  a  whirl.  But  the 
whirl  is  clockwise,  or  the  reverse  of  the  whirl  of  the  cyclone; 
hence  it  is  known  in  Weather  Bureau  cant  as  the  "anticyclone." 
The  winter  anticyclone,  therefore,  is  usually  a  cold  wave. 

The  high  of  the  winter  cold  wave  is  an  area  of  considerable 
pressure.  Usually  the  barometer  stands  above  30.50  inches; 
occasionally  it  mounts  nearly  to  31.00  inches.  For  this  reason 
the  cold  air  spreads  far  south — sometimes  carrying  freezing 
weather  far  into  Florida,  to  the  detriment  of  the  semitropical 
orchards.  The  southern  part  of  Florida  is  the  only  part  of 
the  United  States  which  escapes  freezing  weather. 

In  many  respects,  the  cold  wave  is  one  of  the  most  valuable 
health  assets  of  the  United  States.  Should  the  ground  be 
covered  with  snow,  so  that  gale  winds  pick  up  no  dust,  it  brings 
the  purest  air  that  mortals  on  land  ever  breathe.  Even  if  the 


WEST  INDIAN  HURRICANES 


153 


ground  is  bare,  the 
high  pressure  invades 
the  nooks  and  cran- 
nies where  foul  air  and 
putrefaction  lurk,  and 
drives  them  out.  The 
cold  wave  with  its 
stinging  wind  is  the 
greatest  scavenger  in 
existence. 

West  Indian  Hurri- 
canes.— The  West  In- 
dian hurricanes  do  not 
differ  materially  from 
other  cyclonic  storms 
in  general  principles, 
and  they  differ  from 
the  typhoons  of  the 
China  Sea  in  name  and 
place  only.  They  are 
cyclonic  storms  of  very 
great  violence  and,  with 
the  exception  of  torna- 
does, they  are  the  most 
destructive  storms  that 
reach  any  part  of  the 
United  States.  The 
wave  that  covered 
Galveston,  the  floods 
that  many  times  have 
swept  the  Sunderbunds 
of  India,  and  the  storm 
that  caused  Isle  Der- 
nier to  melt  away  were 
hurricanes  of  the  cy- 
clonic type  —  whirling 
up  draughts  toward 
which  the  surface  wind 
blew  from  every  direc- 
tion. 


154 


THE   DAILY   WEATHER   MAP:    STORMS 


WEST  INDIAN  HURRICANES 


155 


The  West  Indian  hurricanes  originate  in  tropical  latitudes, 
somewhere  north  of  the  equator.  They  move  in  a  northwesterly 
direction  until  they  reach  the  latitude  of  westerly  winds;  then 
they  recurve  and  move  in  a  northeasterly  course.  In  some  in- 
stances a  hurricane  recurves  before  reaching  the  Florida  coast; 
in  others  it  advances  until  the  recurve  crosses  the  Gulf  of  Mexico. 
In  the  first  instance  it  is  not  likely  to  cover  anything  more  than 
the  coast  plain ;  in  the  second 
the  storm  center  may  sweep 
the  eastern  United  States 
from  the  Gulf  to  the  St. 
Lawrence  valley.  After  re- 
curvature,  hurricanes  move 
more  rapidly — occasionally  as 
much  as  50  miles  per  hour. 

These  storms  are  called 
West  Indian  hurricanes  from 
the  fact  that  they  are  first 
noted  at  a  West  Indian 
weathej  station,  frequently  at 
Barbados.  They  sometimes, 
originate  far  to  the  eastward 
of  the  West  Indies,  sometimes 
in  the  Caribbean  Sea.  Since 
vessels  are  now  fitted  with 
radio-telegraphic  apparatus, 
hurricanes  are  commonly 
reported  before  reaching  a 
land  weather  station.  Once 
discovered,  their  movements 
are  closely  watched  and  are 

made  known  to  shipping  until  they   disappear    in    the    North 
Atlantic. 

The  dead  calm  of  tropical  seas  is  the  real  beginning  of 
the  West  Indian  hurricane.  The  air,  moist  almost  to  the  dew- 
point,  is  heated  next  the  surface  until  it  becomes  more  buoyant 
than  the  colder  air  above  it.  Finally  the  unstable  equilibrium 
is  overcome  and  an  updraught  occurs.  The  warm  air  of  the  up- 
draught  is  chilled  by  its  expansion  and  its  moisture  is  condensed. 
The  latent  heat  thus  set  free  adds  to  the  strength  of  the  up- 


-w  sw.7s  sw.1 
From  Redway's  Physical  Geography. 

Storm  cards  showing  movement  of  wind 
in  West  Indian  hurricane. 


156       THE  DAILY  WEATHER  MAP:  STORMS 

draught,  and  the  cyclonic  movement  quickly  develops  into  a 
hurricane  of  tremendous  energy.  Hurricane  winds  at  Galveston 
were  estimated  to  have  a  velocity  of  125  miles  per  hour;  100 
miles  per  hour  was  registered  before  the  anemometer  was  blown 
away. 

According  to  Chief  Forecaster  E.  H.  Bowie,  U.  S.  Weather 
Bureau,  if  a  West  Indian  hurricane,  moving  westward  in  the 
longitude  of  eastern  Cuba,  is  north  of  the  island,  it  will  recurve 
east  of  Florida,  provided  an  area  of  high  pressure  covers  the 
northwestern  states.  But  if  the  hurricane  is  moving  westward 
over  Cuba  or  the  western  Caribbean  Sea  when  an  area  of  low 
pressure  occupies  the  northwest,  and  the  pressure  is  high  in 
the  eastern  states,  the  storm  will  probably  move  to  the  Gulf 
of  Mexico  and  reach  the  Gulf  Coast  after  recurving. 

Form  and  Dimension  of  Cyclonic  Storms. — Extended  meas- 
urements of  the  areas  of  low  and  of  high  pressure,  made  by 
Loomis  and  based  on  the  isobars  of  the  daily  weather  map, 
showed  them  to  be  elliptical  in  form,  the  longer  axis  usually 
pointing  a  little  east  of  northeast.  The  average  dimensions 
were  found  to  be  1600  miles  on  the  long  axis  by  about  one-half 
of  that  extent  along  the  short  axis.  The  average  dimension  of 
anticyclones  is  about  the  same.  These  values  apply  pretty 
closely  to  the  dimensions  of  the  cyclonic  storms  of  western 
Europe. 

The  low  of  the  West  Indian  hurricanes  is  very  much  smaller 
in  area.  Even  after  its  existence  has  been  discovered  it  may 
not  be  more  than  100  miles  in  diameter;  and  by  the  time  it 
passes  a  West  Indian  weather  station  it  may  not  be  more  than 
200  or  300  miles  across.  After  it  recurves  and  enters  the  United 
States,  its  area  is  much  less  than  that  of  the  ordinary  cyclonic 
storm;  the  isobars  are  usually  regular  and  more  nearly  of 
circular  shape  than  those  of  ordinary  storms.1 

Storm  Probabilities. — Before  storm  forecasts  were  sent  to 
vessels  by  radio-telegraphy,  the  sailing  master  of  the  vessel 
was  obliged  to  rely  upon  himself  for  weather  predictions.  He 
based  his  forecasts  on  his  barometer,  clouds,  and  the  wind.  A 
close  study  of  these  enabled  him  to  make  forecasts  that  were 

1  It  is  not  unlikely  that  the  eccentricity  of  the  ellipse  of  the  cyclonic  storm 
depends  on  the  velocity  of  the  whirl — the  higher  the  wind-velocity,  the 
more  nearly  it  approaches  a  circular  form. 


STORM  PROBABILITIES  157 

marvelously  good.  With  intelligent  study  of  wind,  clouds,  and 
moisture,  one  should  be  able  to  forecast  most  ordinary  weather 
changes  from  eight  to  twelve  hours  in  advance,  without  the  aid 
of  barometer  or  weather  map.  This  does  not  apply  to  such  local 
disturbances  as  tornadoes,  thunder-storms  and  hail,  nor  to 
such  conditions  as  ice  storms  and  sleet. 

Throughout  the  greater  part  of  the  United  States  easterly 
winds  indicate  the  approach  of  a  cyclonic  storm.  If  the  wind 
is  from  the  south  or  the  southeast,  the  storm  is  probably  ap- 
proaching along  a  path  to  the  north  of  the  observer;  if  the  wind 
has  settled  to  a  quarter  between  east  and  northeast,  the  track  is 
somewhere  south  of  the  observer;  if  the  wind  is  due  east,  the 
observer  is  probably  in  or  near  the  track  of  the  storm  center. 

If  the  sky  remains  clear  with  an  easterly  wind  the  rain  area 
is  likely  to  pass  some  distance  from  the  observer;  but  if  the 
sky  becomes  gray,  and  then  white,  and  the  air  perceptibly 
damper,  rain  is  not  likely  to  be  far  away.  When  cirro-stratus 
clouds  appear  in  the  easterly  sky,  rain  or  snow  is  pretty  certain 
at  hand  within  a  few  hours. 

The  position  of  the  storm  center  may  be  determined  by 
watching  the  wind  closely  and  noting  any  change  that  may  occur. 
Standing  with  the  back  to  the  wind  the  area  of  low  pressure  is 
on  the  left  hand,  and  the  area  of  high  pressure  on  the  right 
hand.  During  the  passage  of  the  storm,  if  the  wind  shifts  from 
the  east  through  north  to  northwest — that  is,  if  it  "backs  in" 
—the  cyclone  center  is  passing  to  the  south  of  the  observer. 
If  it  veers  through  the  south  to  the  west  or  the  northwest,  the 
storm  center  is  passing  north  of  the  observer. 

The  cooperative  observer  can  do  much  to  aid  in  establishing 
definite  facts  on  which  predictions  may  be  made.  Among  them 
and  of  first  importance  is  establishment  of  the  direction  of  rain- 
winds.  These,  as  has  been  shown,  are  easterly  winds,  but 
conditions  of  topography  may  change  the  real  direction  to  one 
that  is  apparent.  The  apparent  direction  should  be  established 
for  each  month  in  the  year.  In  every  community  there  are 
weather-wise  people  who  possess  valuable  information  that  they 
have  not  recorded.  Such  information  should  be  considered 
carefully  and  accepted  or  rejected  as  the  case  may  be. 

The  number  of  days  in  each  month  on  which  o.oi  inch  or 
more  of  rain  has  fallen  should  be  noted,  tabulated,  and  com- 


158  THE   DAILY   WEATHER   MAP:    STORMS 

pared  with  the  map  of  rain  frequency  published  by  the  Weather 
Bureau.  From  this  table  a  coefficient  of  the  probability  of  rain- 
fall for  the  particular  station  may  be  deduced  by  dividing  the 
number  of  rainy  days  by  365,  or  366,  as  the  case  may  be.  In 
a  similar  manner,  the  probability  of  rain  for  each  month  may  be 
established.  It  is  pertinent  to  add,  however,  that  forecasts 
made  from  such  coefficients  are  by  no  means  certain ;  often  they 
are  disappointing. 

The  average  duration  of  rainfall  may  be  deduced  by 
dividing  the  total  number  of  hours  during  which  rain  has  fallen 
for  the  month  by  the  number  of  rainy  days.  If  the  duration  is 
to  be  based  on  the  average  length  of  storms,  the  number  of 
storms  may  *be  taken  as  the  divisor.  In  the  northeastern  part 
of  the  United  States  the  average  duration  of  rainstorms  is  five 
hours;  in  the  southeastern  part,  four  hours;  in  the  western 
highland  region,  including  the  plains,  about  three  hours;  and 
in  the  basin  region  probably  not  more  than  one  hour.  The 
intensity,  or  rate  of  rainfall  per  hour,  is  a  matter  of  great  im- 
portance. It  is  tabulated  at  regular  intervals  at  Weather 
Bureau  stations. 

It  is  well  to  bear  in  mind  that  the  artificial  production  of  rain 
is  a  delusion.  No  appreciable  fall  of  rain  will  occur  unless  a 
continued  updraught  of  air  is  produced,  and  neither  cannonading 
nor  explosions  at  a  considerable  height  has  accomplished  this. 
Possibly  the  conjunction  of  planets  may  affect  the  movement 
and  the  formation  of  storms;  if  so,  however,  the  connection  has 
not  been  established. 

Secondary  Storms;  Tornadoes. — When  a  whirlpool  forms  in 
a  stream,  smaller  whirlpools  almost  always  occur  near  its  edge. 
These  secondary  whirls  result  from  the  formation  of  the  larger 
whirl.  Similarly,  secondary  whirls  of  the  air  are  very  apt  to 
accompany  the  cyclonic  storms  which  pass  over  the  Great 
Lakes  and  down  the  St.  Lawrence  Valley.  In  the  winter  the 
secondary  storms  thus  formed  appear  along  the  Virginia  coast, 
or  perhaps  to  the  north  of  it,  and  move  north  or  northeast  with 
heavy  snow  squalls  and  high,  gusty  winds.  In  the  summer 
they  are  attended  by  hailstorms,  thunder-storms  and  tornadoes. 
These  occur  usually  on  the  south  or  the  southeast  side  of  the 
low.1 

1  In  the  southern  hemisphere  they  form  on  a  northerly  quadrant. 


SECONDARY  STORMS:  TORNADOES  159 

Tornadoes  are  less  frequent  than  thunder-storms,  but  they 
are  the  most  violent  and  destructive  storms  that  come  into  the 
experience  of  humanity.  The  term  is  loosely  applied  to  almost 
every  violent  wind;  it  is  incorrectly  applied  to  any  secondary 
storm  that  is  not  a  true  whirlwind,  or  ' "twister."  There  are  no 
definite  conditions  known  by  which  tornadoes  may  be  fore- 
cast; but  when  the  path  of  a  northerly  storm  dips  southward 
and  increases  in  intensity,  tornadoes  are  likely  to  occur. 

The  tornado  is  a  whirling  storm,  and  the  whirl  becomes  so 
rapid  that  the  vortex  develops  into  a  black  funnel-cloud.  The 
funnel  is  usually  observed  first  in  the  air.  As  the  whirl  in- 
creases, the  funnel  gradually  extends  downward  to  the  ground. 
No  measured  velocity  of  the  whirl  is  known  to  have  been 
made ;  but  calculations  based  on  the  weight  and  the  surface  of 
bodies  moved  by  the  wind  show  that  the  velocity,  in  various 
instances,  has  exceeded  500  miles  per  hour.1 

The  first  visible  warning  of  the  tornado  is  the  gathering  of 
a  bank  of  very  dense  cloud,  usually  in  a  westerly  quadrant — 
southwest,  west  or  northwest.  The  color  of  the  cloud  bank 
varies.  Not  infrequently  it  appears  much  like  the  smoke  from 
a  burning  hay  barn,  or  a  strawstack;  quite  frequently  it  is  a 
dark  greenish  gray.  The  color  depends  on  the  position  of  the 
observer  with  reference  to  the  sun.  The  cloud  bank  is  always 
in  tumultuous  commotion  within  itself. 

It  is  in  this  cloud  bank  that  the  funnel  of  the  tornado  forms. 
In  some  instances,  as  the  tip  of  the  funnel  approaches  the  ground, 
an  inverted  funnel  is  formed  at  the  ground,  quickly  joining  the 
funnel  hanging  from  the  cloud.  The  funnel  is  the  destructive 
part  of  the  tornado.  It  uproots  trees,  or  twists  their  trunks 
to  the  breaking  point.  Wherever  the  tornado  passes  through 
woodlands  its  path  is  marked  by  uprooted,  shattered  and 
twisted  trunks  of  trees.  When  the  funnel  strikes  a  building 
the  latter  bursts  outwardly.  In  various  instances  a  roof  has 
been  carried  in  fragments  a  distance  several  miles  away.  Wooden 
railway  bridges  have  been  dismembered  and  splintered  beyond 
repair,  and  steel  bridges  have  been  torn  from  their  abutments 
and  crumpled  into  shapeless  heaps.  Chickens  have  been  al- 
most completely  plucked;  straw  and  twigs  have  been  driven 

1  This  was  computed  by  Bigelow  in  the  case  of  the  Missouri  tornado  of 
May  27,  1896, 


160 


THE   DAILY  WEATHER  MAP:    STORMS 


SECONDARY  STORMS;    TORNADOES 


161 


endways  into  boards ;  and  large  animals  have  been  lifted  and 
carried  considerable  distances.  In  one  case  a  cow  was  lifted 
out  of  a  high  corral  and  deposited,  not  seriously  injured,  several 
hundred  feet  away.1 

The  cause  of  the  tornado  cannot  always  be  determined; 
in  a  few  instances  it  has  been  assumed  by  reason  of  strong 
circumstantial  evidence.  During  the  passage  of  a  northerly 


W 


The  graphic  story  of  a  tornado. 

cyclone  that  has  bent  its  path  into  the  south,  great  volumes  of 
dry,  cold  air  are  sometimes  whirled  into  regions  where  the  air  is 

1  The  accompanying  diagram  illustrates  a  gruesome  story.  When  the 
funnel  cloud  approached  the  house,  the  family  fled.  At  first  they  ran  north- 
ward, a  direction  of  safety.  Then,  one  after  another,  they  turned  eastward 
and  ran  into  the  whirl.  The  younger  of  two  girls  ran  directly  into  the  tornado 
path  and  was  instantly  killed.  The  mother  had  reached  a  place  of  safety; 
then  she  turned  into  the  tornado  path  and  was  crushed  to  death  against  a 
tree  trunk.  The  older  girl  and  a  boy  also  turned  toward  the  storm  track; 
their  clothing  was  stripped  from  them  and  they  were  torn  and  bruised.  The 
father,  with  the  baby  in  his  arms,  had  reached  a  place  of  safety;  then,  in 
fright,  he  too  ran  back  into  the  storm  track  where  both  were  killed.  This  is 
one  story  that  illustrates  many.  As  a  rule,  the  path  of  safety  is  toward  the 
northwest  or  the  southeast  if  the  direction  of  the  tornado  track  can  be  de- 
termined. 


162  THE   DAILY  WEATHER   MAP:    STORMS 

warm  and  moist.  Now,  if  the  heavier  cold  air  rests  at  the  sur- 
face of  the  earth  no  disturbance  follows.  On  the  other  hand, 
if  the  cold  air  comes  to  rest  on  the  top  of  a  thick  layer  of  warm 
air,  an  unstable  condition  results.  Sooner  or  later  an  updraught 
of  warm  air  takes  place  and  tornado  conditions  are  established. 
The  rapid  whirl  creates  a  near- vacuum,  and  this  accounts  for 
the  fact  that  buildings  struck  bv  the  funnel-cloud  burst  out- 
wardly. 

The  air  movement  of  tornadoes  is  three-fold — the  up- 
draught, the  whirl,  and  the  progressive  movement.  The  de- 
structive path  of  the  tornado  is  as  wide  as  the  funnel-cloud, 
rarely  more  than  a  few  rods.  The  entire  whirl  is  not  much 
more  than  half  a  mile  in  diameter;  the  extent  of  the  path  varies 
from  a  few  miles  to  about  200  miles.  The  tornado  progresses 
along  its  track  at  a  rate  varying  from  10  or  12  miles  an  hour 
to  express-train  speed.  The  funnel-cloud  is  formed  at  a  height 
of  about  half  a  mile. 

From  the  nature  of  the  case,  the  best  values  concerning 
the  dynamic  force  of  the  tornado  are  only  approximate,  but 
even  these  are  instructive.  Normal  air  pressure  is  at  the 
rate  of  2117  pounds  per  square  foot.  Now,  if  the  air  pressure 
within  the  funnel  is  only  three-fourths  normal  when  the  funnel 
involves  a  building,  the  air  pressure  inside  the  building  will  be 
530  pounds  per  square  foot  greater  than  on  the  outside.  Such  a 
difference  in  pressure  is  sufficient  to  burst  the  walls  of  almost 
any  building. 

Tornadoes  are  most  prevalent  in  May,  June  and  July; 
the  average  of  these  months  exceeds  that  of  the  rest  of  the 
year.  They  are  more  common  in  the  United  States  than  in 
Europe.  The  regions  of  greatest  frequency  are  the  lower  Ohio 
and  Missouri  valleys  and  the  Central  Mississippi  region.  Very 
few  occur  in  the  arid  region  west  of  the  one-hundredth  meridian 
and  fewer  still  are  reported  north  of  the  fiftieth  parallel.1 

Hail  and  electrical  discharges  frequently  accompany  tor- 
nadoes, but  they  have  nothing  to  do  with  the  cause  of  them; 
and  although  the  updraught  occurs  in  thunder-storms — and 
probably  a  cyclonic  movement  of  the  air  within  it — one  is 

1  Sir  Napier  Shaw,  of  the  Meteorological  Office,  London,  does  not  even 
mention  tornadoes  in  his  "  Forecasting  Weather."  They  are  unknown  in  the 
British  Isles,  the  line  squall  being  its  nearest  approach. 


SECONDARY  STORMS;    TORNADOES  163 

hardly  warranted  in  considering  the  tornado  as  an  exaggerated 
thunderstorm. 

Desert  Whirlwinds. — Dust  spouts  are  common  in  desert 
regions.  A  little  after  sunrise  during  warm  weather,  the  still 
air  next  to  the  ground  becomes  very  much  warmer  than  the 
air  at  the  distance  of  a  few  hundred  feet  above  the  ground. 
In  time  the  unstable  equilibrium  is  upset  and  chimneys  of 
updraught  are  formed  here  and  there,  carrying  columns  of 
fine  dust  to  a  height  of  several  hundred  feet.  At  a  distance 
the  dust  columns  are  strongly  outlined.  When  the  cold  air  has 
settled  to  the  ground  the  whirl  and  its  dust  column  ceases. 
Later  in  the  day,  the  setting  in  of  a  steady  wind  puts  an  end 
to  the  unequal  warming  of  the  air. 

Apache  Indians  have  made  use  of  the  desert  whirls  as  signals, 
creating  them  by  setting  fire  to  the  spines  of  a  columnar  cactus 
that  is  common  in  the  southwestern  states.  The  burning  of  the 
spines  at  the  right  moment  made  enough  heat  to  start  the  up- 
draught. When  the  warm  air  at  the  surface  has  been  pressed 
upward  the  descending  air  is  perceptibly  colder  at  times. 

Waterspouts. — If  the  whirl  of  the  updraught  over  water 
increases  to  a  velocity  whereby  the  skin  friction  of  the  wind 
overcomes  the  cohesion  of  the  water,  a  waterspout  is  formed. 
The  whirl  of  the  updraught  is  strong  enough  to  whisk  the  water 
into  the  air,  at  the  same  time  whirling  it  into  a  mist.  Un- 
doubtedly some  of  the  water  drawn  into  the  air  is  vaporized. 
When  the  spout  breaks,  a  considerable  part  of  the  water  in  the 
air  drops  in  a  torrential  deluge.  Popular  tradition  has  it  that 
sea  water  drawn  into  a  spout  falls  as  fresh  water — a  tradition 
that  is  contrary  to  the  facts  of  the  case. 

White  squalls  are  fair-weather  whirlwinds  over  the  water. 
In  many  instances  there  is  not  enough  condensation  in  the  air 
to  form  a  cloud;  occasionally,  however,  a  bit  of  misty  cloud, 
the  "bull's  eye,"  is  "visible.  At  the  surface,  the  wind  is  strong 
enough  to  whisk  the  water  into  white  spray,  but  the  whirl  is 
not  strong  enough  to  draw  it  into  the  updraught. 


CHAPTER  XIV 
FORECASTING    THE  WEATHER:    WEATHER   FOLKLORE 

Two  classes  of  people  criticize  Weather  Bureau  forecasts, 
the  public  and  the  Weather  Bureau.  Probably  the  Weather 
Bureau  itself  is  the  severer  critic  of  the  two.  Its  rules  for 
purposes  of  verification  are  inflexibly  definite.  The  practise  is 
definite  as  to  the  character  of  the  forecast,  the  time  of  occurrence, 
and  the  place  of  occurrence. 

Rain — Fair. — By  rain  in  this  connection  is  meant  any  kind 
of  precipitation  in  season.  The  general  term  precipitation  is 
used  to  embrace  rain,  snow,  sleet  or  hail;  but  in  forecasts, 
"rain"  may  be  used  to  cover  any  or  all.  To  verify  a  "rain" 
forecast,  precipitation  must  occur  to  the  amount  of  o.oi 
inch  or  more.  The  forecast  may  designate  "rain,"  "showers," 
"thunder-storm,"  "snow,"  "sleet,"  etc.,  but  the  meaning  for 
verification  does  not  vary.  Even  the  term  "clearing,"  when 
used  in  connection  with  a  rain  forecast,  means  that  rain  will 
fall  during  a  part  of  the  time  covered  by  the  forecast. 

For  purposes  of  verification,  fair  means  only  the  absence  of 
precipitation.  The  forecaster  may  differentiate  the  kinds  of 
fair  weather  to  be  anticipated  as  partly  cloudy,  cloudy,  un- 
settled, overcast,  or  threatening — these  are  all  variations  of 
the  forecaster's  "fair."  If  precipitation  occurs  to  the  amount 
of  o.oi  inch  or  more,  by  the  rules  of  the  Weather  Bureau  the 
forecast  fails. 

Warmer — Colder.— The  rules  concerning  temperature  fore- 
casts are  also  equally  definite,  but  with  certain  limits  in  veri- 
fication. If  the  forecast  is  "warmer,"  any  rise  of  temperature 
is  a  verification;  so  also  is  "cooler"  if  lower  temperature  is  fore- 
cast. But  if  a  change  is  not  forecast,  or  if  the  words  "not  much 
change,"  "slight  change,"  "continued  warm,"  (or  cool),  or 
"stationary  temperature"  are  used,  a  definite  number  of  de- 
grees (6  in  summer  and  10  in  winter)  is  required  to  vitiate  the 

164 


TIME  AND  PLACE  OF  OCCURRENCE  165 

forecast.      Modifying  words,    "slightly,"    "much,"    "probably," 
etc.,  do  not  relieve  the  forecaster  of  the  failure  of  his  verification. 

Time  of  Occurrence. — The  forecasts  most  generally  sent 
out  for  publication  are  based  on  the  8:00  A.M.  observations  and 
reports.  The  terms  designating  time  are  "to-night"  and  the 
name  of  the  following  day.  "To-night"  covers  the  twelve-hour 
period  from  8:00  P.M.  of  the  current  day  to  8:00  A.M.  of  the 
following  day.  Therefore,  whatever  is  forecast  for  "to-night" 
must  occur  .within  these  time  limits.  "Rain  to-night"  would 
fail  of  verification  if  none  occurred  until  after  8:00  A.M.  the 
day  following,  even  though  a  heavy  downpour  set  in  immedi- 
ately thereafter.  'The  following"  day  begins  at  8:00  A.M. 
and  ends  at  8.00  P.M.  after  the  current  day — that  is,  for  purposes 
of  verifying  the  8.00  A.M.  forecast  on  Monday,  "Tuesday"  covers 
only  that  portion  of  the  day  between  8.00  A.M.  and  8.00  P.M. 

Place  of  Occurrence. — Most  forecasts  are  made  to  cover 
individual  states.  The  larger  states  are  subdivided  into  "north," 
"south,"  "east"  and  "west"  sections.  The  daily  forecast  may 
be  for  the  whole  of  a  state  or  for  any  of  its  sections.  If  rain 
is  forecast,  say,  for  New  Jersey,  and  none  is  reported  from  any 
of  the  stations  in  the  state,  the  verification  fails,  even  though 
showers  may  have  occurred  at  nearby  stations  in  Pennsylvania 
and  New  York. 

The  Value  of  Safety. — Measured  by  their  effect  on  commerce, 
production,  and  transportation,  some  weather  changes  are  of 
no  particular  effect;  they  are  neither  beneficent  nor  hurtful. 
Other  changes  are  classed  as  "critical";  if  they  occur  unex- 
pectedly— that  is,  without  forewarning,  they  may  result  in  loss 
by  damage,  or  by  destruction;  they  also  may  cause  human 
suffering. 

The  "unexpected"  may  be  unseasonable  rains,  snowstorms, 
floods,  frosts,  cold  waves,  hot  spells,  tornadoes,  or  other  severe 
weather.  These  are  the  weather  conditions  to  which  the  fore- 
caster must  be  keenly  alert;  they  are  the  possibilities  that 
demand  his  chief  care.  Forecasters  realize  that  it  is  wiser  to 
warn  against  a  killing  frost  that  does  not  materialize  than  to 
fail  in  warning  against  one  that  does  appear.  The  unverified 
forecast  of  frost  may  cause  some  trouble  and  some  loss,  but  the 
killing  frost  that  comes  without  warning  is  likely  to  result  in 
loss  infinitely  greater. 


166  FORECASTING   THE   WEATHER:     FOLKLORE 

In  the  raisin-growing  regions  of  California,  a  shower  on  the 
fruit  curing  in  the  open  air  causes  very  great  damage.  The  fruit 
grower,  therefore,  is  closely  observant  of  the  forecast  of  showers. 
The  expense  of  stacking  his  trays,  however,  is  small  compared 
with  the  loss  of  his  crop  or  the  impairment  of  its  quality,  result- 
ing from  a  shower.  It  is  to  the  credit  of  the  district  forecaster 
that  in  many  years  not  a  shower  has  occurred  of  which  timely 
warning  was  not  given. 

Recently  a  West  Indian  hurricane  threatened  the  Gulf 
Coast  and  warnings  were  duly  issued.  Precautions  were  taken 
as  indicated;  but,  by  the  time  the  hurricane  reached  the  Gulf 
Coast,  not  much  energy  was  left  in  it.  But  what  would  have 
been  the  result  had  the  warnings  been  omitted  and  the  hurri- 
cane had  possessed  the  violence  of  the  storms  which  destroyed 
Galveston  and  Corpus  Christi? 

It  is  the  desire  of  the  Weather  Bureau  to  prevent  loss  by  fore- 
warning. The  district  forecasters  do  not  strain  points  for  high 
percentages  of  verification.  A  row  of  failures  may  be  dis- 
couraging; a  mistake  against  the  forecaster  may  make  him  a 
target  of  derision;  but  a  mistake  which  results  in  loss  of  life  is 
irreparable.  Therefore,  in  making  forecasts,  it  is  "safety  first." 

Those  who  make  intelligent  use  of  Weather  Bureau  predic- 
tions realize  that  forecasts  are  not  insurance  policies.  They 
merely  are  expressions  which  represent  the  experience  and  judg- 
ment of  the  best-trained  meteorologists.  In  one  particular  the 
dissemination  of  weather  forecasts  might  be  made  even  more 
valuable — namely,  by  issuing  a  map  and  forecasts  based  on 
the  8:00  P.M.  reports,  to  be  published  in  the  morning  papers. 
When  the  public  decides  that  it  really  wants  this  information, 
the  information  will  be  forthcoming.  As  a  rule,  the  public 
gets  what  it  deserves,  but  not  always  what  it  needs. 

POPULAR  WEATHER  PREDICTION — FOLKLORE  l 

Weather  prediction  is  probably  as  old  as  human  history  and 
some  of  the  sayings  popular  to-day  passed  current  more  than 
three  thousand  years  ago.  They  survive  because  they  are  true. 
Mariners  at  sea  and  shepherds  on  land  learned  their  lessons 

1  The  material  for  much  of  this  chapter  is  inspired  by  Professor  Edward 
Garriott's  Weather  Folklore,  published  by  the  U.  S.  Weather  Bureau. 


BAROMETRIC  INDICATIONS  J67 

well;  neither  the  one  nor  the  other  was  possessed  of  a  daily 
weather  map.  The  wind  was  a  fair  barometer;  the  blinking  of 
the  stars  was  an  excellent  hygrometer.  The  discovery  of  the 
underlying  principles  of  barometric  pressure  was  the  beginning 
of  modern  meteorology.  The  use  of  the  barometer  quickly 
appealed  to  sailors,  and  practically  every  deep-water  vessel  in 
the  world  is  equipped  with  one.  Transportation  companies, 
lighting  companies,  farmers  and  manufacturers  find  it  a  neces- 
sity. The  invention  of  the  aneroid  barometer  has  popularized 
its  use  tremendously. 

In  the  hands  of  one  without  experience,  or  without  training 
in  the  use  of  it,  the  barometer  is  usually  a  disappointment. 
To  the  trained  observer,  or  to  the  observer  who  has  gained  wis- 
dom by  experience,  it  is  an  instrument  of  the  highest  value. 
To  be  serviceable  in  forecasting  weather  conditions  it  must  be 
watched — not  casually  but  systematically.  The  experience 
that  comes  from  intelligent  study  of  pressure  changes  will 
enable  an  observer  to  command  most  gratifying  results. 

General  Pressure  Indications. — As  a  rule,  pressure  changes 
should  not  be  considered  by  themselves;  they  should  be  studied 
in  conjunction  with  changes  in  temperature,  humidity  and 
wind  direction.  There  are,  however,  certain  general  weather 
conditions  indicated  by  changes  in  barometric  pressure  which 
hold  good: 

A  gradual  rise  of  the  barometer  indicates  settled  fair  weather. 

A  rise  from  a  very  low  pressure  indicates  wind  and  clearing  weather — 
the  more  rapid  the  rise,  the  stronger  the  wind. 

Rapid  changes  in  pressure  indicate  early  and  marked  changes  in  the 
weather. 

A  sudden  rise  in  pressure  indicates  as  great  a  change  as  a  sudden  fall. 

The  wind  is  apt  to  blow  hardest  when,  after  having  been  very  low,  the 
barometer  begins  to  rise. 

Should  the  pressure  continue  to  remain  low  after  the  sky  has  cleared, 
expect  more  rain  within  twenty-four  hours. — PRINCE. 

If  the  pressure  falls  two  or  three  tenths  of  an  inch  in  four  hours  or  less, 
expect  gale  winds. — PRINCE. 

In  summer  a  sudden  fall  in  pressure  indicates  a  thunder-storm;  if  it  does 
not  rise  when  the  storm  ceases,  unsettled  weather  may  be  expected. 

A  fall  in  pressure  not  accompanied  by  stormy  conditions  indicates  a  severe 
storm  at  a  distance. 

A  steady  but  very  slow  fall  in  pressure  indicates  that  the  low  and  its  storm 
conditions  is  moving  slowly.  "Long  falling,  long  last;  short  notice,  soon 
past." — FITZROY. 


168  FORECASTING   THE   WEATHER:     FOLKLORE 

During  a  period  of  low  pressure,  fine  weather  may  be  regarded  with  sus- 
picion; a  change  may  be  expected  at  any  time  and  most  likely  it  will  be 
sudden. 

The  barometer  falls  lower  for  high  winds  than  for  rain, 
but  torrential  rains  may  accompany  a  very  low  pressure.  In 
winter,  if  high  temperature  accompanies  very  low  pressure, 
heavy  rain  followed  by  a  cold  wave  may  be  expected. 

A  rising  barometer  usually  indicates  winds  having  a  westerly 
element — southwest,  west,  or  northwest.  A  falling  barometer 
usually  indicates  winds  having  an  easterly  element — south- 
east, east,  or  northeast.  The  rule  is  not  infallible,  however. 
Occasionally  there  occurs  a  dry  east  wind  with  a  rising  barometer. 

A  gradual  but  steady  fall  of  the  barometer  indicates  unsettled 
weather,  increasing  moisture  and  rain.  A  slow  fall  from  a 
very  high  barometer  indicates  unsettled  and  rainy  conditions 
rather  more  certainly.  A  sudden  and  rapid  fall  indicates  a 
sudden  downpour  and  high  winds,  or  both.  In  summer  a 
thunder-storm  is  preceded  usually  by  a  drop  in  pressure. 

Wind  Indications  of  Weather  Conditions. — Throughout 
the  eastern  half  of  the  United  States  l  winds  with  a  westerly 
element — northwest,  west,  and  southwest  winds— indicate  fair 
weather.  Winds  with  an  easterly  element — northeast,  east,  and 
southeast  winds — indicate  unsettled  weather,  rain  or  snow. 

A  straight  north  wind  is  apt  to  be  a  clear- weather  wind. 
"The  north  wind  driveth  away  rain." — PROV.  xxv,  23;  but 
this  is  not  always  true,  especially  if  it  veers  into  the  north- 
east. 

Straight  south  winds  along  the  Atlantic  and  Gulf  Coasts 
are  apt  to  bring  unsettled  weather,  inasmuch  as  the  south 
wind  blows  from  the  sea,  it  usually  brings  warm  air  and  excess- 
ively humid  weather.  Occasionally  it  brings  storm  conditions. 

West  winds  are  dry  winds ;  in  the  eastern  half  of  the  United 
States  they  are  apt  to  be  dust-laden  also.  In  midsummer  they 
blow  many  miles  over  sun-heated  ground  and  they  are  there- 
fore apt  to  be  hot  winds. 

East  winds  almost  always  precede  rain  and  snow  by  twenty- 
four  hours  or  more.  Along  the  Atlantic  Coast  the  east  wind  is 
pretty  certain  to  be  a  storm-breeder. 

1  The  narrow  strip  along  the  Gulf  Coast  should  be  excepted  from  the 
general  rule.  In  summer  it  is  within  the  Trade  Wind  belt. 


BAROMETER  AND  WIND  INDICATIONS 


169 


Northwest  winds  are  the  prevailing  winds  of  the  greater  part 
of  the  United  States.  They  are  also  the  clearing  winds  for  most 
of  the  cyclonic  storms  that  sweep  the  country;  they  constitute 
practically  all  the  cold-wave  winds. 

Southwest  winds  are  the  prevailing  winds  during  the  summer 
months  in  the  eastern  part  of  the  United  States.  With  a  falling 
barometer,  they  bring  rain. 

Northeast  winds  are  storm -winds;  almost  always  they  are 
cold  and  raw. 

Southeast  winds  are  rain  winds  along  the  entire  coast  and 
much  of  the  interior  of  the  United  States,  for  the  greater  part 
of  the  year.  The  time  varies  from  twelve  to  eighteen  hours  in 
winter  and  from  eighteen  to  thirty-six  hours  in  summer. 

Barometer  and  Wind  Indications. — When  pressure  and 
wind-direction  are  both  considered  and  interpreted  according 
to  their  mutual  relations,  local  forecasts  can  be  made  with  a 
much  greater  degree  of  certainty.  During  the  colder  months, 
throughout  the  United  States,  the  western  highlands  excepted, 
precipitation  begins  with  falling  pressure.  In  the  summer 
months,  and  in  the  western  highlands  it  is  apt  to  begin  with 
the  rising  barometer.  The  following  indications  have  been 
compiled  for  the  Weather  Bureau  by  Garriott. 


29 . 80  or  below,  rising  rapidly . 
30 .  oo  or  below,  rising  slowly . . 

30. 10  to  30.20,  rising  rapidly. 


30.10  to  30.20,  steady.  . 


30.20  and  above,  steady 

29 . 80  or  below,  falling  rapidly . 
29 . 80  or  below,  falling  rapidly . 

30.00  or  below,  falling  rapidly, 
30 .  oo  or  below,  falling  slowly . 


w  Clearing  and  colder. 

s  to  sw       Clearing  within  a  few  hours; 

fair  for  several  days. 

sw  to  nw  Fair,  followed  in  two  days  by 
warmer  and  unsettled 
weather. 

sw  to  nw  Fair,  with  stationary  tem- 
perature for  one  or  two 
days. 

sw  to  nw    Continued  fair;    steady  tem- 
perature, 
e  to  n        Severe  northeast  gales;  heavy 

rain  or  snow. 

s  to  e  Severe  storm  probable,  fol- 
lowed by  clearing  and 
colder  weather. 

se  to  ne  Rain  with  high  wind,  followed 
within  24  hours  by  clearing 
and  colder  weather. 

se  to  ne  Rain  likely  to  continue  48 
hours, 


170 


FORECASTING   THE   WEATHER:     FOLKLORE 


30. 10  or  above,  falling  rapidly.  .  .        e  to  ne 


30. 10  or  above,  falling  slowly.  ...        e  to  ne 


30. 10  to  30.20,  falling  slowly.  ...  se  to  ne 
30. 10  to  30.20,  falling  rapidly.  .  .        s  to  se 

30. 10  to  30.20,  falling  slowly.  ...        s  to  se 

30. 10  to  30.20,  falling  rapidly.  .  .  sw  to  nw 

30. 10  to  30.20,  falling  slowly.  ...  sw  to  nw 


Rain  or  snow  probable  within 
12  to  24  hours.  In  winter, 
snow  and  high  winds. 

In  summer,  light  winds  and 
rain  after  48  hours;  in 
winter,  rain  within  24  hours. 

Rain  in  12  to  1 8  hours. 

Increasing  wind;  rain  in  12  to 
24  hours. 

Rain  within  24  hours. 

Warmer;  rain  in  18  to  24 
hours. 

Warmer;  rain  in  24  to  36 
hours. 


Barometer  and  Temperature  Indications. — The  following 
are  noted  by  P.  R.  Jameson.  They  apply  chiefly  to  that  part 
of  the  United  States  and  Canada  east  of  the  Rocky  Mountains, 
approximately  from  the  latitude  of  the  Ohio  River  to  that  of 
the  Saskatchewan  River. 


PRESSURE  RISING 


Below  30°  F 

Between  30°  F  and  40°  F. 
Between  40°  F  and  50°  F. 
Between  50°  F  and  60°  F. 


Cold  wave 

Freezing  temperature 

Frost  or  freezing  temperature  probable 

Cooler 


Above  60°  F t Warm;   cool  nights 


PRESSURE  FALLING 


Below  30°  F 

Between  30°  F  and  40°  F ... 
Between  40°  F  and  50°  F..  .  , 
Between  50°  F  and  60°  F..  . 
Above  60°  F 


Overcast ;   snow 

Rain,  sleet  or  snow 

Unsettled;   rain 

Heavy  rains 

Showery  conditions;   unsettled 


Humidity  Indications. — The  gathering  moisture  of  the  air 
or,  technically,  its  increasing  humidity,  is  an  indication  of  un- 
settled weather.  Ordinarily,  the  air  which  may  be  at  the  dew- 
point  at  daylight  becomes  relatively  dry  at  midday  because 
its  -higher  temperature  gives  it  what  is  popularly  termed  "a 
greater  capacity  for  moisture."  But  if  the  relative  humidity 
remains  high  in  the  middle  of  the  day  it  is  evident  that  the 
absolute  humidity  has  increased,  and  unsettled  weather  may  be 
expected. 


HUMIDITY  AND  WEATHER  CONDITIONS  171 

A  hygrometer  is  useful  in  detecting  increase  of  moisture, 
but  it  is  not  wholly  essential.  Where  a  hygrometer  is  available 
it  is  apparent  that  the  less  the  difference  between  the  wet  bulb 
and  the  dry  bulb  the  greater  the  moisture  content  of  the  air 
and  the  greater  certainty  of  unsettled  weather 

The  effects  of  increasing  moisture  in  the  air  are  so  well  known 
that  the  literature  of  them  is  great,  and  popular  sayings  con- 
cerning them  are  found  in  all  ages. 

When  the  locks  turn  damp  in  the  scalp  house  most  surely  it  will  rain. — 
INDIAN  TRADITION. 

If  metal  plates  sweat  it  is  a  sign  of  foul  weather. — PLINY. 

The  tightening  of  cordage  on  ships  is  taken  by  sailors  as  a  sign  of  ap- 
proaching rain. 

A  red  sun  has  water  in  his  eye. — NEW  ENGLAND  TRADITION. 

When  it  is  evening,  ye  say  it  will  be  fair  weather,  for  the  sky  is  red ;  and 
in  the  morning  it  will  be  foul  weather,  for  the  sky  is  red  and  lowering. — 
MATTHEW  xvi,  2-3. 

Rainbow  in  the  morning,  shepherds  take  warning; 

Rainbow  at  night,  shepherds'  delight. 

Circles  around  the  sun  or  the  moon  indicate  increasing  moisture. 

Salt  absorbs  moisture  quickly.  Its  becoming  coherent  is  a  sign  of  increas- 
ing moisture. 

The  sunflower  lifts  its  head  when  the  moisture  of  the  air  increases. 

The  perfume  of  flowers  becomes  stronger  when  the  air  becomes  damp; 
so  also  does  the  odor  of  a  tobacco  pipe. 

It  is  well  to  bear  in  mind  that  these  traditions  apply  to 
a  more  or  less  sudden  change  from  dry  to  moist  air,  and  not 
to  the  long-continued  spells  of  moisture  that  come  with  steady 
sea  winds. 

Moist  weather  of  long  duration  may  be  clear,  as  is  com- 
monly the  case  along  the  Atlantic  Coast  in  summer;  but  a 
rapid  change  from  dry  to  moist  air  almost  always  brings  hazy 
conditions,  and  this  is  the  sort  of  change  that  precedes  rain. 

And  if  through  mists  Sol  shoots  his  sullen  beams, 
Frugal  of  light  in  loose  and  straggling  streams, 
Suspect  a  drizzling  day  and  southern  rain 
Fatal  to  fruits,  and  flocks  and  promised  grain. 

— VIRGIL. 

The  foregoing  are  only  a  few  of  the  traditions  and  folklore 
sayings  concerning  the  humidity  of  the  air.  Nearly  all  of  them 
may  be  reduced  to  one  or  the  other  of  two  general  principles — 


172  FORECASTING   THE   WEATHER:    FOLKLORE 

the  hygroscopic  character  of  many  common  substances,  or  the 
mistiness  of  the  air  which  tends  to  scatter  all  but  the  red  rays 
of  the  sun. 

The  varying  conditions  of  humidity  usually  afford  indications 
more  or  less  characteristic.  These  are  more  noticeable  at  morn- 
ing and  evening  when  the  humidity  is  high.  They  are  apt  to 
be  most  pronounced  on  the  horizon  when  vision  penetrates  a 
layer  of  air  of  greatest  density.  The  following  are  proverbs  of 
seamen : 

A  whitish-yellow  western  sky  indicates  rain. 

Unusual  hues  of  the  sky  forming  a  background  of  sharply  edged  clouds 
indicate  heavy  rains  and  gusty  winds. 

A  white,  yellow  or  greenish-yellow  sunset  indicates  a  storm. 

A  diffuse  or  hazy  sunset  indicates  a  coming  storm. 

A  purple  sky  foretells  continued  fine  weather. 

A  blur  or  haziness  at  the  horizon  indicates  unsettled  weather. 

This  may  be  correct  if  the  haziness  is  due  to  misty  air;  but 
it  is  not  true  if  the  hazy  or  distorted  outlines  are  caused  by  the 
refraction  of  air  currents. 

If  the  sun  draws  water  in  the  morning  expect  rain  by  night. 

This  may  be  true  in  some  localities,  but  it  is  not  generally 
true.  The  appearance  is  due  to  the  reflection  of  straggling 
rays  of  sunlight  from  dust  motes  or  from  mist. 


Red  evening  sky,  a  fine  to-morrow. 
A  red  morn  betokens  a  tempest. 


Cloud  Indications. — Cloud  matter  is  the  first  step  in  con- 
densation. If  precipitation  is  feeble  in  energy  and  slow  in 
process,  only  cloud  is  formed;  if  it  is  more  energetic  the  cloud 
matter  forms  rain  or  snow. 

Because  cirro-stratus  clouds,  higher  than  others,  consist  of 
the  first  precipitation  on  the  advancing  low,  they  are  among 
the  best  indications  of  an  approaching  storm.  In  a  majority 
of  instances  they  are  the  overflow  from  the  upper  part  of  the 
approaching  cyclone;  they  may  be  more  than  one  hundred 
miles  in  advance  of  the  storm  center. 

Not  all  cirrus  and  cirro-stratus  clouds  are  storm  clouds, 
however.  If  they  rise  to  a  higher  altitude,  or  if  they  disappear, 
fine  weather  is  likely  to  follow.  If  they  accompany  a  rising 
barometer,  fair  weather  is  likely  to  continue. 


CLOUDS  AND  WEATHER  -  173 

Cirro-stratus  clouds  covering  the  western  horizon  to  a 
height  of  30  degrees  or  more  indicate  rain  within  twelve  hours 
as  a  rule.  This  is  a  still  more  certain  indication  if  the  lower  edge 
of  the  cloud  is  wavy.  A  cirrus  patch  with  streamer  edges,  which 
increases  in  size,  indicates  snow. 

Clouds  moving  apparently  against  a  surface  wind  in  reality 
are  moving  with  an  upper  current  of  the  air.  That  is,  cross- 
winds  are  blowing,  and  cross-winds  very  commonly  precede 
rain  or  snow;  a  departing  storm  may  also  clear  with  cross- 
winds. 

When  ye  see  a  cloud  rise  out  of  the  west,  straightway  ye  say :  There  cometh 
a  shower;  and  so  it  is. — LUKE  xii,  54. 

The  greasy,  gray  clouds  which  are  characteristic  of  tropical 
skies  during  the  rainy  season  are  sometimes  seen  during  summer 
in  northern  latitudes.  They  are  pretty  certain  to  indicate  a 
heavy  downpour. 

Greenish-tinted  masses  of  cloud  collecting  in  the  southeast  indicate  heavy 
rains. 

A  mackerel  sky — twelve  hours  dry. 

Rain  from  high  clouds  or  from  thin  clouds  does  not  last  long. 

If  detached  clouds  increase  in  size  the  moisture  of  the  air 
also  is  increasing;  if  they  decrease  in  size  and  disappear  the 
moisture  is  decreasing.  While  the  former  condition  in  itself 
does  not  indicate  an  approaching  storm,  it  is  instructive  in 
connection  with  other  local  indications. 

A  sky  overcast  with  high  clouds  does  not  indicate  stormy  conditions  if 
the  clouds  remain  high.  If  the  pressure  falls  and  the  clouds  lower,  stormy 
weather  may  be  expected. 

Rapidly  increasing  cumulus  clouds  indicate  thunder-storms.  A  thunder- 
head  or  high  cauliflower  top  to  a  cumulus  cloud  denotes  a  rapid  updraught, 
which  in  itself  is  the  beginning  of  a  thunder-storm. 

Still  and  very  slowly  moving  cumulus  clouds  indicate  a  continuance  of 
fair  weather. 

A  cloud  layer  against  the  side  of  a  mountain  range,  if  rising  to  a  greater 
height,  indicates  increasing  pressure;  if  dropping  lower,  decreasing  pressure. 

Cirro-stratus  together  with  alto-stratus  clouds  indicate  precipitation  with 
a  probability  of  about  90  per  cent. — McAoiE. 

Animal  and  Plant  Indications. — To  assert  that  four-footed 
animals,  birds,  and  insects  sometimes  foretell  approaching 
weather  conditions  is  to  make  a  very  radical  claim  which  cannot 


174  FORECASTING   THE   WEATHER:     FOLKLORE 

be  established.  But  to  assert  that  they  do  not  recognize  existing 
conditions  and  respond  to  them,  and  to  weather  changes  in 
progress,  is  to  fly  in  the  face  of  the  experience  of  four  thousand 
years. 

To  most  animal  life  weather  conditions  are  of  even  greater 
importance  than  they  are  to  humanity.  If  the  experience  of 
naturalists,  and  of  those  who  are  in  close  contact  with  herds 
and  with  bees  is  worth  anything,  one  must  admit  that  nature 
has  provided  them,  not  with  "prophetic  instinct,"  but  with 
keener  sensitiveness  to  changes  in  weather  conditions  than  is 
possessed  by  human  beings.  The  bison  is  especially  sensitive 
to  weather  changes. 

All  shepherds  agree  that  before  a  storm,  sheep  become  frisky,  leap  and 
butt  one  another.— FOLKLORE  JOURNAL. 

When  horses  and  cattle  become  restless  and  uneasy,  a  change  to  un- 
settled weather  may  be  expected. 

When  fowls  oil  their  feathers  and  are  unusually  noisy,  unsettled  weather 
may  be  expected. 

A  bee  was  never  caught  in  a  shower. 

When  bees  hover  about  their  hives  and  refuse  to  take  flight,  unsettled 
weather  may  be  expected. 

When  house  flies  bite,  expect  rain.1 

When  spiders  strengthen  their  webs  rain  may  be  expected. 

The  song  of  the  robin  bringeth  rain. 

The  odor  of  plants  of  the  nightshade  family  becomes  very  rank  with  the 
approach  of  rain. 

Milkweed  closing  at  night  indicates  foul  weather. 

The  convolvulus  derives  its  name  from  the  fact  that  its 
flower  closes  when  a  rapid  increase  of  moisture  occurs.  This  is 
true  also  of  the  pimpernel.  The  pitcher  plant,  on  the  other 
hand,  opens  to  receive  the  coming  shower.  The  leaves  of  the 
sugar  maple,  the  cottonwood  and  the  sycamore  turn  so  as  to 
show  the  under  side  on  the  approach  of  a  shower.  Occasionally 
this  is  noticeable  in  the  case  of  clover. 

Experience  will  teach  the  observer  the  value  of  popular 
weather  signs  and  traditions.  The  experience  of  out-of-door 
men  whose  employments  are  affected  by  weather  conditions 
should  not  be  tossed  lightly  aside.  Perhaps  the  explanation  of 
their  reasoning  may  not  bear  critical  analysis;  the  results, 

!This  is  not  true  of  the  house  fly.  The  biting  stable  fly,  however, 
seeks  shelter  indoors  on  the  approach  of  stormy  weather. 


INDICATIONS  OF  HEAVENLY  BODIES  175 

on  the  other  hand,  are  apt  to  have  a  high  value.  Weather 
science  now  has  treatises  of  inestimable  value,  but  no  book 
from  which  weather  knowledge  may  be  obtained  surpasses 
wind  and  sky. 

Indications  of  Heavenly  Bodies. — It  is  hardly  necessary  to 
note  that  such  indications  are  due  to  the  effects  of  the  varying 
moisture  content  of  the  air,  together  with  slight  refractions 
and  diffractions  caused  by  the  moisture  of  the  air. 

Red  sun  in  the  morning,  let  the  shepherd  take  warning. 

A  circle  around  the  sun  foretells  foul  weather. 

The  circle  of  the  sun  wets  the  shepherd. 

A  mock  sun  brings  rain. 

The  moon  with  a  circle  brings  water  in  her  beak. 

A  lunar  halo  indicates  rain;  the  larger  the  halo  the  sooner  may  rain  be 
expected. 

A  large  ring  around  the  moon,  and  low  clouds,  rain  will  follow  in  twenty- 
four  hours;  a  small  ring  and  high  clouds,  rain  in  several  days. 

The  halo  around  the  sun  or  the  moon  is  neither  more  nor  less 
than  a  very  faint  rainbow  caused  by  the  refraction  of  light 
rays  as  they  pass  through  mist  or  very  thin  cloud  matter.  It 
is  therefore  a  phenomenon  of  humidity. 

Before  the  rising  of  a  wind  the  fainter  stars  are  not  visible,  even  on  a 
clear  night. — PLINY. 

Mixed  air  currents  cause  so  much  refraction  of  light  that 
feeble  points  of  light  are  not  perceived.  With  clear,  still  air  the 
stars  are  very  bright.  In  astronomical  observatories,  observa- 
tions made  on  windy  nights  have  but  little  value,  so  great  is  the 
blurring  from  refraction.  The  higher  the  power  of  the  telescope, 
the  greater  the  impairment  of  visibility. 


PART  II 


CHAPTER  XV 

THE  MEASUREMENT  OF  TEMPERATURE: 
THERMOMETERS 

Quantitative  measurements  in  temperature,,  based  upon  the 
calorie,  have  a  definite  place  in  physics;  and  those  based  on  the 
British  thermal  unit  have  a  broad  application  in  various  econo- 
mies. Human  sensitiveness  to  heat  does  not  pertain  to  quantity 
but  to  intensity.  A  large  block  of  ice  at  30°  may  contain  more 
heat,  quantitatively  considered,  than  a  red-hot  horseshoe.  If 
carried  into  a  room  whose  air  was  far  below  freezing,  the  ice 
might  warm  the  room  to  a  greater  degree  than  would  the  horse- 
shoe. Humanity,  and  indeed,  all  living  things  require  the 
intensity  of  heat  that  enables  living  organisms  to  function 
naturally  and  properly.  The  vital  questions  therefore  are — 
"How  warm  is  it?" — or,  "How  cold  is  it?" — or,  "Is  physical 
comfort  satisfied?"  These  conditions  depend  upon  intensity  of 
heat — that  is,  upon  temperature. 

Temperature. — The  term  temperature  has  a  broad  applica- 
tion. It  is  an  expression  of  the  varying  warmth  of  earth,  air  and 
water,  with  relation  to  life. 

The  unit  of  temperature  is  a  degree,  a  measured  part  of  the 
expansion  which  a  column  of  mercury  within  a  tube  undergoes 
when  heated  from  the  melting  point  of  ice  to  the  boiling  point 
of  water  at  sea  level.1 

1  The  standard  conditions  are  somewhat  complex.  The  real  test  in  ther- 
mometry  is  a  comparison  of  a  thermometer  in  the  process  of  manufacture 
with  an  accurately  made  standard.  A  standard  thermometer  is  a  part  of 
the  equipment  of  the  manufacturer. 

177 


178     MEASUREMENT   OF   TEMPERATURE:    THERMOMETERS 

Three  scales  of  degree  measurements  are  more  or  less  in 
use.  In  the  Reaumur  scale,  now  rarely  used,  the  expansion  is 
divided  into  80  parts;  in  the  centigrade,  into  100  parts;  in 
the  Fahrenheit,  into  180  parts.  In  the  Reaumur  and  the  centi- 
grade scales  the  zero,  o°,  of  temperature  is  at  the  melting  point 
of  ice;  in  the  Fahrenheit,  32  degrees  below  it.  The  centigrade 
scale  is  used  chiefly  in  Continental  Europe.  The  degree  values 
are  inconveniently  large  and,  in  many  cases,  fractional  units 
of  the  degree  must  be  expressed.  Winter  temperatures  require 
the  use  both  of  positive  and  negative  quantities,  and  this  adds 
to  the  labor  of  computation  a'nd  to  the  likelihood  of  error. 

Absolute  Temperature. — The  fact  that  neither  the  centi- 
grade nor  the  Fahrenheit  scale  per  se  expresses  the  relation  of 
the  volume  of  a  gas  to  its  temperature  has  led  to  the  establish- 
ment of  a  theoretical  absolute  zero  of  temperature.  The  following 
demonstration  and  the  accompanying  cut  explain  the  method 
of  its  determination.  A  glass  tube  about  50  inches  long  and 
closed  at  one  end  contains  a  free-moving  piston  of  mercury, 
resting  about  20  inches  from  the  open  end  of  the  tube.  The 
tube  is  first  placed  in  a  container  filled  with  melting  ice.  When 

-273°  0°  100° 

The  empiric  determination  of  the  absolute  zero  of  temperature. 

the  piston  has  reached  the  low  point  its  position  is  marked. 
It  is  then  transferred  to  a  container  of  boiling  water,  and  the 
position  of  the  mercury  piston  is  again  marked.  The  amount 
of  expansion  is  divided  into  180  equal  parts  or  units.  If,  now, 
the  distance  between  the  first  mark  and  the  lower  end  of  the 
tube  be  measured,  it  will  be  found  to  contain  almost  precisely 
459  of  these  units.  Each  unit  corresponds  to  I  degree  Fahren- 
heit. Hence  from  this  experiment  absolute  or  natural  zero 
would  be  —459.4°.  On  the  centigrade  scale  it  is  —273.13°. 
Absolute  temperature  is  commonly  expressed  as  A°;  or  45940A. 
The  natural  zero  deduced  from  an  investigation  of  the  pres- 
sure of  a  gas  at  constant  volume  has  the  same  scale  value  as 
the  absolute  zero.  It  is  inferred,  in  consequence,  that  the 
absolute  or  natural  zero  is  the  temperature  at  which  molecular 
motion  ceases. 


THE  THERMOMETER  179 

The  Thermometer. — The  ordinary  thermometer  consists  of 
a  bulb  or  reservoir  fused  to  the  end  of  a  glass  tube  or  stem. 
The  tubing  from  which  the  stem  is  drawn  is  a  wedge-shaped 
prism  with  a  strip  of  white  enamel  fused  to  the  convex  surface. 
When  the  prismatic  tube  is  drawn  out  into  thermometer  stems 
the  wedge  angle  becomes  a  lens  which  magnifies  the  fine  thread 
of  mercury;  the  enamel  becomes  an  opaque,  white  background 
against  which  the  thread  of  mercury  is  plainly  visible.  The 
bulbs  of  ordinary  thermometers  are  commonly  blown  at  the  end 
of  the  drawn  tube.  Those  of  the  best  thermometers  are  made 
of  a  specially  constructed  glass  and  are  fused  to  the  end  of  the 
drawn  tube. 

Most  solids,  in  cooling  from  fusion  or  from  intense  heating, 
suffer  what  is  known  as  ' 'hysteresis"— that  is,  molecular  changes 
continue  for  a  considerable  time.  These  changes  alter  the  size 
of  the  bore  of  the  tube.  In  order  to  overcome  them,  the  tubes 
of  thermometers  of  the  highest  grade  are  laid  away  to  "season" 
or  "temper"  for  a  period  of  two  years.  The  shrinkage  of  an  un- 
seasoned tube  is  likely  to  cause  the  readings  to  register  as  much 
as  6  degrees  too  high. 

The  bore  of  the  thermometer  is  microscopic  in  diameter; 
in  thermometers  graduated  to  fractions  of  a  degree,  it  may  be 
less  than  o.ooi  inch;  ordinarily  it  is  from  0.002  inch  to  0.005 
inch.  In  the  construction  of  precision  thermometers  the  bore 
is  measured  under  the  microscope,  and  a  bulb  of  hard  glass  of 
the  required  size  is  fused  to  the  end  of  the  tube.  Cylindrical 
bulbs  are  preferable  to  spherical  bulbs;  they  present  a  greater 
surface  and  therefore  are  more  sensitive.  The  expansion  and 
contraction  of  the  glass  with  changes  of  temperature  is  somewhat 
greater  than  with  spherical  bulbs,  but  thermometer  scales  are 
compensated  for  this  correction. 

The  bulb  of  the  thermometer  is  usually  filled  with  mercury 
at  the  time  it  is  fused  to  the  stem.  While  hot,  the  open  end  of 
the  stem  is  inserted  in  a  vessel  containing  pure  mercury.  As 
the  air  in  the  bulb  cools,  its  contraction  causes  a  small  quantity 
of  mercury  to  be  forced  through  the  bore  of  the  stem  into  the 
bulb.  The  mercury  in  the  bulb  is  then  heated  to  its  boiling  point 
and  the  open  end  of  the  stem  again  dipped  into  the  mercury. 
This  process  is  repeated  until  both  bulb  and  steam  are  com- 
pletely filled. 


180     MEASUREMENT  OF    TEMPERATURE:    THERMOMETERS 

The  mercury  and  the  tube  are  apt  to  contain  a  minute 
quantity  of  moisture.  If  this  remains  it  is  pretty  certain  to 
cause  a  broken  column  of  mercury  in  the  tube,  thereby  rendering 
the  thermometer  imperfect.  To  prevent  this,  the  filled  tube  is 
kept  for  some  hours  at  a  temperature  above  the  boiling  point 
of  water.  The  "roasting"  requires  care  and  experience. 

When  the  roasting  process  is  completed  the  bulb  is  again 
heated  and  as  soon  as  the  mercury  is  expanded  to  the  top  of 
the  stem  the  latter  is  sealed,  leaving  an  angle  or  "hook"  at  the 
top.  The  hook  holds  the  tube  fast  to  the  scale. 

The  tube  is  now  ready  to  be  graduated.  For  this  purpose 
it  is  placed  successively  in  brine  at  2°,  melting  ice  at  32°,  and 
water  baths  at  62°  and  92°.  The  position  of  the  top  of  the 
column  is  marked  for  each  temperature  and  usually  for  each 
tenth  degree  of  the  scale.  An  engine  ruling  machine  divides 
each  division  into  30  parts.  The  division  marks  are  also  scaled 
below  2°  and  above  92°.  The  metal  scale  is  subdivided  according 
to  the  marks  on  the  stem. 

Several  manufacturers  produce  three  grades  of  instruments. 
On  those  of  the  first  class  minute  spots  may  be  found  at  the 
various  testing  temperatures.  If  the  marks  do  not  appear, 
the  thermometer  is  not  trustworthy  for  precision  purposes. 
Weather  Bureau  thermometers  are  specially  tested,  and  a 
certificate  showing  the  error  for  each  10  degrees  accompanies 
each  instrument.  A  thermometer  which  does  not  comply  with 
these  requirements  is  not  a  standard  instrument. 

Thermometers  of  the  second  grade  are  not  engraved  on  the 
stem;  the  divisions  are  on  the  metal  scale  to  which  the  tube  is 
attached.  The  tubes  are  seasoned;  but  trifling  inaccuracies 
unfit  them  for  use  where  precision  is  required.  They  are  suf- 
ficiently accurate  for  ordinary  uses. 

Thermometers  of  the  third  grade  are  "rejects"  and  rarely 
bear  the  maker's  name.  The  inaccuracy  is  always  more  than 
I  degree,  and  it  is  likely  to  vary  in  different  parts  of  the  scale. 
In  many  instances  these  thermometers  are  sold  to  jobbers 
and  retailers  who  stamp  fictitious  names  on  them.  Experience 
and  practise  will  usually  enable  one  to  discover  the  maker. 
The  scope  of  thermometer  scales  varies  greatly.  Practically 
every  industry  employs  temperature  measurements  which 
require  specially  constructed  thermometers. 


WEATHER  BUREAU  THERMOMETERS 


181 


Weather  Bureau  Thermometers. — Several  kinds  of  ther- 
mometers are  necessary  for  the  requirements  of  a  weather 
service;  and  these  are  practically  the  same  in  all  parts  of  the 
world  where  a  weather  service  is  maintained.  In  weather  sta- 
tions the  daily  maximum  and  the  daily  minimum  are  required; 
a  continuous  graphic  record  is  desirable,  but  the  instrument  for 
this  purpose  is  supplied  to  stations  of  first-class  equipment  only. 

A  standard  thermometer  of  the  ordinary  type,  that  is,  one. 
which  shows  existing  temperature  at  any  time,  is  desirable. 
A  minimum  thermometer  can  be  used  for  this  purpose,  but  a 
standard  instrument  is  preferable.  The  scale,  divided  to  single 
degrees,  should  be  engraved  on  the  tube  and  on  the  metal 


Maximum  and  minimum  registering  thermometers. 
Weather  Bureau  patterns. 


strip  as  well.  Readings  are  made  to  the  nearest  degree  mark. 
If  the  fraction  is  exactly  half  a  degree  the  preceding  figure,  if 
odd,  will  be  increased  by  I  degree;  if  even,  it  will  remain 
unchanged.1 

The  maximum  thermometer  is  so  called  from  the  fact  that  the 
mercury  in  the  tube  is  not  drawn  back  into  the  bulb  when  the 
temperature  lowers.  The  expansion  of  the  mercury  in  the  bulb 
forces  the  flow  into  the  bore,  as  with  ordinary  thermometers. 
A  slight  constriction  of  the  bore  at  the  top  of  the  bulb  prevents 
a  backflow,  thereby  leaving  the  mercury  in  the  bore  at  the 
maximum  temperature  since  its  last  previous  setting.  Usually 
the  maximum  temperature  of  the  day  occurs  between  2 130  P.M. 
and  4:00  P.M.  and  the  thermometer  should  be  set  late  in  the 

1  This  rule  applies  in  all  Weather  Bureau  computations. 


182     MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 

evening  or  early  next  morning  so  as  to  record  the  maximum  for 
the  next  day. 

;  To  insure  accuracy  of  readings  the  thermometer  should  not 
be  higher  than  the  eye  of  the  observer;  and  to  avoid  error  of 
refraction  by  the  lens  front,  the  observer  should  stand  squarely 
in  front  of  the  end  of  the  mercury  column.  An  error  of  refraction 
may  amount  to  half  a  degree. 

The  "setting"  of  the  maximum  thermometer  is  accomplished 
by  whirling  it  around  a  stud  and  bearing  at  the  end  opposite 
the  bulb;  centrifugal  motion  forces  the  mercury  past  the  stric- 
ture, and  back  into  the  bulb. 

The  maximum  thermometer  of  the  Weather  Bureau  type 
should  rest  in  a  nearly  horizontal  position,  the  bulb  slightly 
higher  than  the  farther  end  of  the  tube.  Ordinarily  this  will 
prevent  any  back  flow.  Occasionally,  however,  a  maximum 
thermometer  fails  to  leave  the  mercury  in  the  bore  at  the  point 
of  maximum  expansion;  for  reasons  not  fully  explained  the 
column  of  mercury  is  drawn  back  toward  the  bulb  as  the  tem- 
perature falls.  A  maximum  thermometer  of  this  sort  is  known 
in  Weather  Bureau  cant  as  a  "retreater."  The  retreating  of 
the  column  of  mercury  may  be  overcome  by  a  slight  increase  in 
the  elevation  of  the  bulb.  When,  however,  it  is  discovered  that 
the  retreating  is  habitual,  the  thermometer  should  be  returned 
to  the  maker. 

Some  maximum  thermometers  set  easily;  others  require  to 
be  whirled  vigorously.  Observers  differ  in  their  likes  and  dis- 
likes. If  the  column  of  mercury  moves  very  easily  there  is 
always  danger  of  error.  If  the  tube  is  held  in  a  horizontal 
position  there  is  the  possibility  of  a  slight  retreat  of  the  column. 
And  if  the  bulb  is  elevated  there  is  always  the  possibility  that 
it  may  slide  toward  the  far  end  of  the  tube.  The  maximum 
thermometer  readings  are  not  trustworthy  for  any  except 
maximum  temperatures.  When  set,  it  should  agree  with  the 
reset  minimum  thermometer. 

It  is  advisable  to  bring  the  thermometer  very  carefully  to 
a  vertical  position  when  the  reading  is  made.  This  corrects 
at  once  any  sliding  of  the  column  that  may  have  occurred. 
But  there  is  always  danger  that  a  part  of  the  mercury  in  the 
tube  may  flow  into  the  bulb  when  the  thermometer  is  brought  to 


MAXIMUM  AND  MINIMUM  THERMOMETERS  183 

a  vertical  position;  this  is  most  likely  to  occur  in  hot  weather 
when  the  column  is  long. 

Taking  everything  into  consideration,  a  maximum  ther- 
mometer that  requires  a  moderately  vigorous  whirling  is  prefer- 
able. The  instrument  with  which  the  observer  can  work  to 
best  advantage  is  the  one  with  which  he  should  be  provided. 

The  minimum  thermometer  is  for  the  purpose  of  registering 
the  lowest  temperature  between  settings.  It  is  exposed  in  a 
nearly  horizontal  position  with  the  bulb  slightly  higher  than 
the  opposite  end  of  the  tube.  Inasmuch  as  winter  minima 
are  sometimes  lower  than  the  freezing  temperature  of  mercury, 
and  because  the  instrument  here  described  requires  a  trans- 
parent column,  minimum  thermometers  usually  contain  alcohol 
instead  of  mercury.  The  alcohol  of  American  Weather  Bureau 
thermometers  usually  is  colored;  in  most  foreign  made  instru- 
ments it  is  uncolored.  In  the  matter  of  visibility  the  gain  of 
a  colored  liquid  is  material,  but  coloring  matter  is  not  essential. 
In  spite  of  care,  a  precipitation  of  the  coloring  matter  occasion- 
ally may  occur,  and  this  is  likely  to  cause  a  slight  constant  error. 
The  space  in  the  tube  above  the  alcohol  contains  air,  more  or 
less  saturated  with  alcohol  vapor. 

The  essential  feature  of  the  minimum  thermometer  is  a 
small  black  index  within  the  bore  and  also  within  the  liquid. 
As  a  lowering  temperature  contracts  the  liquid  column,  the 
cohesion  of  its  surface  drags  the  index  toward  the  bulb.  When 
the  liquid  expands,  however,  it  flows  around  the  index  without 
moving  it.  The  index  therefore  shows  the  lowest  temperature 
between  settings.  The  minimum  temperature  is  read  at  the 
end  of  the  index  farthest  from  the  bulb.  The  temperature  may 
be  read  from  the  minimum  thermometer  at  any  time,  reading 
from  the  end  of  the  column  of  alcohol,  and  not  from  the 
index. 

The  minimum  thermometer  is  usually  attached  to  a  strip  of 
brass,  bent  so  that  the  instrument  is  held  about  an  inch  from 
the  board  support.  It  is  fastened  so  that  the  end  containing 
the  bulb  may  be  swung  to  an  inverted  position.  The  maximum 
thermometer  is  fastened  to  the  same  support.  The  stud  on 
which  it  whirls  is  about  2  inches  long.  The  free  end  rests  on  a 
pin  which  is  removed  when  the  thermometer  is  set. 


184     MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 

Instead  of  the  fasteners  described  above,  clamps,  sometimes 
called  the  Townsend  supports,  may  be  used  to  hold  the  ther- 
mometers. The  clamps  are  fastened  to  the  board  support  and 
permit  the  setting  of  the  thermometers.  The  clamps  are  issued 
as  a  part  of  station  equipment. 

Care  and  Adjustment  of  Thermometers. — The  thermometers, 
being  exposed  to  the  weather,  accumulate  dust;  the  metal 
parts  may  become  tarnished  or  even  rusty.  It  is  advisable  to 
use  a  soft  camel's  hair  brush  for  removing  the  dust,  and  this 
should  be  done  two  or  three  times  a  week.  When  occasion 
requires,  a  polishing  brush  may  be  used  on  the  metal  parts. 
It  is  more  desirable  to  prevent  than  to  remove  rust  and  tarnish. 

Unless  the  maximum  thermometer  becomes  a  retreater, 
it  is  not  likely  to  get  out  of  order.  Even  if  drifting  snow  blown 
into  the  shelter  incrusts  it,  no  damage  is  likely  to  result.  It  is 
better  to  allow  the  snow  to  melt  off  than  to  attempt  to  remove 
it  by  force. 

If  a  maximum  thermometer  has  not  been  set  for  a  long 
time,  a  break  in  the  column  which  refuses  to  unite  may  result. 
The  same  may  occur  if  the  moisture  has  not  been  wholly  expelled 
during  the  roasting  process.  In  such  a  case  it  is  usually  possible 
to  drive  the  space  to  the  small  chamber  at  the  end  of  the  tube. 
It  may  be  driven  into  the  bulb;  if  this  is  done  the  break  is 
likely  to  work  back  into  the  column  again.  If  the  instrument  is 
held  in  a  vertical  position,  bulb  down,  at  a  distance  of  I  or  2 
inches  from  a  table,  and  is  allowed  to  fall  with  vertical  blows  so 
as  to  hit  a  thickness  of  blotting  paper  placed  on  the  table,  the 
broken  space  gradually  displaces  the  mercury  until  it  reaches 
the  top  of  the  column. 

A  break  in  the  column  of  mercury  in  the  tube  is  not  neces- 
sarily a  defect;  it  is  only  when  the  break  will  not  close — that  is, 
when  it  leaves  an  open  space — that  error  in  the  reading  results. 
In  such  a  case  the  thermometer  should  be  discarded,  or  else 
returned  to  the  maker  for  repair. 

The  minimum  thermometer  is  usually  out  of  order  when  it 
is  received  at  its  destination.  The  index  may  be  fast  at  the  top 
of  the  tube,  or  in  the  bulb;  most  likely  the  alcohol  column  is 
broken,  a  half  dozen  or  more  bubbles  occurring;  possibly  some 
of  the  alcohol  is  lodged  in  the  chamber  at  the  farther  end  of 
the  tube. 


THERMOMETER  SHELTER  185 

To  loosen  the  index,  tap  the  edge  of  the  metal  scale  with  a 
small  piece  of  wood — say,  a  clothes  pin — until  it  becomes 
free. 

With  bulb  end  down,  let  the  thermometer  fall  vertically 
an  inch  or  more  so  that  it  strikes  endways  on  the  table  or  the 
shelter  floor.  Little  by  little  the  alcohol  will  flow  along  the  tube ; 
the  broken  parts  of  the  column  become  shorter;  and  the  bub- 
bles disappear. 

If  this  fails,  hold  the  thermometer  at  its  upper  end  and  bring 
it  down  forcibly  as  though  striking  hard  blows  with  a  hammer 
— being  careful,  however,  not  actually  to  strike  anything.  It 
may  require  vigorous  exercise,  but  the  centrifugal  force  will 
finally  bring  the  broken  parts  to  the  rest  of  the  column.  It 
usually  requires  from  a  quarter  to  half  an  hour  to  put  a  mini- 
mum thermometer  with  a  broken  column  in  order.  Great  care 
must  be  used  that  no  part  of  the  alcohol  remains  in  the  chamber 
at  the  farther  end  of  the  tube. 

Thermometer  Shelter. — Maximum  and  minimum  ther- 
mometers should  be  sheltered  from  the  sun  and  from  direct 
contact  with  precipitation  of  every  sort.  They  also  must  be 
placed  so  that  they  are  in  contact  with  free  air.  They  must 
be  sheltered  from  heat  radiated  from  buildings,  metal  roofs,  and 
pavements.  The  board  support  should  not  be  attached  directly 
to  the  wall  of  a  building;  if  on  a  porch  it  should  be  attached  to 
an  outrigger  that  leaves  a  space  of  a  few  inches  from  the  build- 
ing. A  wide,  covered  porch  with  a  northerly  or  an  easterly 
exposure  is  the  best  position  about  a  building. 

The  daily  maxima  on  the  south  side  of  a  house,  within  3 
feet  of  the  wall  will  be  from  2  degrees  to  6  degrees  higher  in 
clear  weather  than  those  on  the  north  side,  close  to  the  house. 
The  minima  will  vary  but  little.  If  only  a  window  exposure  is 
available,  a  north-facing  window  should  be  selected,  and  the 
thermometers  should  be  screened  from  the  window  if  the  room 
is  heated.  There  should  be  several  inches  of  space  between  the 
shelter  front  and  the  window. 

In  cities,  the  flat  roof  of  a  building  frequently  offers  the  best 
position  for  thermometers.  A  graveled  roof  reflects  less  heat 
than  a  metal  roof,  and  should  be  preferred  when  possible.  In 
any  case  the  shelter  should  be  placed  where  the  thermometers 
are  not  affected  by  heat  reflected  from  nearby  walls.  The  best 


186     MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 


position   on   the   roof   must   be   determined   by   judgment   and 

experience. 

In   open   country   and   sparsely  built  localities,   the   shelter 

built  after   the   plans   recommended   by   the   Weather   Bureau 

should  be  used.  This 
consists  of  a  minia- 
ture house,  3  by  2 
by  2  feet,  with 
louvered  sides  and  a 
removable  top.  The 
front  may  be  let 
down  when  readings 
are  made.  The  front 
should  face  the 
north.  The  shelter 
rests  on  braced  legs 
which  should  be 
anchored  firmly  to 
the  ground.  The 
top  of  the  shelter  is 
likely  to  become  hot 
enough  to  radiate 
heat  to  thethermom- 
eters.  This  may  be 
prevented  in  part  by 
a  double  roof  with 
an  air  space,  or  by 
covering  the  roof 
loosely  with  asbestos 
cloth  or  with  lino- 
leum. There  can  be 
no  objection  to  plac- 


A  THERMOMETER  SHELTER. 

It  is  placed  in  the  shade  of  tall  trees,  and  receives    ing    the     shelter    in 

direct  sunlight  a  few  hours  in  the  morning  only.       the  shade   of  a  tree 

that  shields  it  from 

the  afternoon    sun,  provided   it    is   not   less   than    8    feet   from 
trunk  and  branches. 

In  locating  a  place  for  a  shelter  it  is  a  good  plan  to  use  a 
second  thermometer  in  various  positions,  checking  and  com- 
paring maxima  and  minima.  Reflection  and  absorption  some- 


ANOMALIES  OF  TEMPERATURE  187 

times  bring  about  unexpected  results.  Observers  with  experi- 
ence are  alert  to  these  possibilities;  the  inexperienced  observer 
must  learn  them.  In  general,  if  the  shelter  is  distant  twice  the 
height  of  an  object  there  will  be  no  errors  caused  by  reflection 
or  by  absorption  from  that  object. 

Anomalies  of  Temperature. — As  a  rule,  minimum  tempera- 
tures— and  they  usually  occur  just'  before  sunrise — are  less  apt 
to  be  affected  by  unusual  conditions  than  are  maximum  tem- 
peratures. The  minimum  temperatures  on  the  south  side  of 
a  building  are  usually  the  same  as  those  on  the  north  side. 
In  prolonged  hot  spells,  however,  this  does  not  always  hold 
true.  The  walls  of  the  southerly  exposure  may  absorb  so  much 
warmth  during  the  day  that  not  all  of  it  is  radiated  at  night. 
As  a  result,  a  minimum  registered  under  such  conditions  will 
be  too  high. 

The  prevalence  of  a  stiff  wind,  especially  the  northwest 
wind  of  cold  waves,  equalizes  temperatures  to  a  remarkable 
extent;  the  minima  of  stations  covering  considerable  areas 
rarely  vary  more  than  I  or  2  degrees.  On  the  other  hand,  on 
very  still  nights  the  minima  of  stations  only  a  few  miles  apart 
may  vary  several  degrees. 

On  very  cold,  still  nights  cold  air  tends  to  settle  by  gravity 
into  low  spots.  This  condition  is  so  marked  that  the  minima 
of  localities  only  a  few  rods  apart  may  vary  as  much  as  2  or 
3  degrees.  This  difference  is  very  noticeable  in  mountain 
valleys  where  the  cold  'air  is  apt  to  flow  down  the  valleys  at 
night.  Frosts  occur  much  more  frequently  along  valley  floors 
than  in  the  foothills  and  the  benches  higher  up. 

City  and  suburban  temperatures  usually  have  about  the 
same  daily  means,  and  their  monthly  averages  should  not  vary 
more  than  a  degree.  The  daily  maxima  and  minima  not  infre- 
quently vary  several  degrees.  This  is  due  chiefly  to  the  fact 
that  the  less  amount  of  smoke  and  dust  in  suburban  localities 
favors  absorption  of  heat  in  the  day  time  and  permits  radiation 
at  night. 

In  early  fall  and  also  in  late  spring,  frost  may  be  observed 
in  sheltered  places  on  the  ground  when  the  thermometer  registers 
several  degrees  above  freezing.  An  observer  may  therefore 
conclude  that  his  minimum  thermometer  is  not  registering  cor- 
rectly. It  is  not  likely  than  an  error  has  occurred;  ground  sur- 


188     MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 

face  temperature,  especially  on  northerly  exposures,  may  register 
several  degrees — on  occasions,  as  many  as  10  degrees — lower 
than  the  thermometers  6  feet  or  more  above  the  surface.1 

Thermometers  on  the  business  streets  of  cities,  especially 
those  in  which  the  blocks  are  solidly  built,  register  from  3  degrees 
to  6  degrees  too  high  as  a  rule,  owing  to  radiation  and  reflection 
from  nearby  buildings.  They  indicate  the  temperature  of  the 
street,  but  not  of  the  free  air. 

The  Six  Maximum  and  Minimum  Thermometer. — A  maxi- 
mum and  minimum  thermometer  of  the  Six  pattern  consists  of 
a  glass  tube  bent  in  two  or  three  sections  as  shown  in  the  accom- 
panying figure.  The  tube  in  the  center  is  a  cylindrical  bulb 
about  o.i  inch  internal  diameter;  the  bulb  at  the  top  of  the 
right-hand  column  is  large  enough  to  have  a  volume  of  about  I 
cubic  centimeter.  The  bore  in  the  U  part  of  the  tube  is  about 
0.02  inch  in  diameter;  it  is  filled  with  mercury.  The  central 
bulb  is  completely  filled  with  a  solution  of  creosote,  or  with 
alcohol.  The  expansion  of  the  liquid  in  the  central  bulb  pushes 
the  mercury  down  on  the  left  side  of  the  U  and  up  on  the  right 
side;  it  also  pushes  liquid  into  the  air  bulb  on  the  right  side, 
slightly  compressing  the  air  and  vapor  in  the  bulb.  Lowering 
temperature  causes  a  contraction  of  the  liquid  in  the  central 
bulb,  thereby  drawing  back  the  mercury  in  the  U.  This  is 
made  more  positive  by  the  compressed  air  and  vapor  in  the 
right-hand  bulb.  The  scale  reads  downward  on  the  left  and 
upward  on  the  right  side.  These  are  marked  respectively  "cold" 
and  "heat,"  or  "night"  and  "day." 

Maxima  and  minima  are  recorded  by  separate  indices 
within  the  bore  of  the  tube.  The  indices  are  pieces  of  steel 
wire  coated  with  glass — in  some  thermometers  they  are  plain 
wire — each  armed  with  two  appendages.  On  one  end  the 
appendage  points  upward;  on  the  other,  downward.  Their 
object  is  to  hold  the  index  lightly  to  the  place  in  the  bore  to 
which  the  mercury  pushes  it.  Pushing  the  indices  is  the  only 
work  the  mercury  in  the  U  tube  performs.  Rising  temperature 
pushes  the  index  in  the  right-hand  tube  upward;  falling  tem- 
perature pushes  the  index  in  the  left-hand  tube  upward.  The 

1  In  the  latitude  of  middle  England  Sir  Napier  Shaw  notes  that  thermom- 
eters on  the  grass  register  lower  by  20  degrees  than  those  in  the  shelter, 
a  difference  of  8  degrees  being  very  common. 


THE  SIX  THERMOMETER 


189 


indices  are  set  by  the  use  of  a  small  magnet  which  accompanies 

the  thermometer.     The  poles  of  the  magnet  are  hollow-ground, 

so  as  to  fit  closely  to  the  tube. 

In  many  respects  the  Six  thermometer  is  preferable  for  ordi- 
nary uses.  It  is  not  so  likely  to  be  broken 
as  the  regular  Weather  Bureau  thermom- 
eters; it  is  very  readily  set;  and  it  is 
more  nearly  "fool-proof"  than  the  delicate 
Weather  Bureau  instruments.  It  is  not  so 
sensitive  as  the  Weather  Bureau  thermom- 
eters; it  is  slow  in  registering;  and  the 
indices  are  occasionally  caught  in  the 
mercury  from  which  they  are  separated 
with  difficulty.  A  violent  jar  may  break 
the  hair-like  appendages  that  cause  the 
indices  to  register.  If  this  happens  to  the 
index  in  the  right-hand  tube  its  repair 
by  a  thermometer  maker  is  possible;  if 
in  the  left-hand  tube  the  case  is  hope- 
less. 

In  selecting  a  thermometer  of  this  type 
one  should  note  first  whether  or  not  the 
readings  of  the  two  tubes  are  the  same. 
When  a  thermometer  has  lain  edgewise, 
or  on  its  side,  for  a  number  of  days — and 
this  may  occur  when  it  is  in  transit  on  a 
railway — a  flow  of  the  liquid  past  the 
mercury,  from  one  tube  to  the  other,  may 
take  place.  As  a  result  the  readings  on 
the  two  sides  do  not  coincide.  An  expert 
in  the  mechanics  of  thermometry  can  make 
the  necessary  adjustment,  but  it  should 

Maximum    and    Mini-  be  done  bY  an  expert  and   not   an  experi- 

mum  Thermometer.      menter.     In    selecting  a  Six   thermometer, 

Six's  pattern  a  comparison  of   the   readings   of   the  two 

sides  should  be  the  first  care. 
The  Six  thermometer  may  not  be  quite  up  to  the  standard 

of , accuracy.     If    the    error    is    small    the    thermometer    needs 

not  be  condemned,  however,   for  an  allowance   can   be   made 

therefor. 


190      MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 

The  Thermograph. — The  thermograph  is  both  a  registering 
and  a  recording  thermometer.  The  essential  part  of  the  mech- 
anism consists  of  two  thin  strips  of  metal  having  different 
coefficients  of  expansion.  The  metal  strips  are  brazed  or 
soldered  surface  to  surface,  bent  to  a  quadrant  or  curled  into  a 
coil,  and  annealed.  The  type  used  by  the  United  States  Weather 
Bureau  consists  of  a  curved  flat  tube  filled  with  mercury  or  with 
alcohol.  In  either  type  of  thermograph  expansion  causes  a 
warping  of  the  metal  which  is  communicated  to  a  lever,  whose 
long  arm  is  a  recording  pen. 

The  recording  part  of  the  thermograph  is  a  drum  containing 
a  clock.  The  clock  is  geared  so  as  to  cause  one  nearly  complete 


Thermograph — nigh  drum. 

revolution  of  the  drum  in  a  week.  The  slight  shortage  of  a 
complete  revolution  is  an  allowance  for  the  margin  of  the 
fastening  of  the  paper  on  which  the  record  is  made. 

The  paper  strips  upon  which  records  are  made  are  about 
12  inches  long.  They  vary  in  width  according  to  requirements. 
Horizontal  lines  lithographed  from  engine-ruled  plates  divide 
the  width  of  the  strip  into  degree  spaces.  Arcs  of  circles,  whose 
radii  are  the  length  of  the  pen,  divide  the  length  of  the  strip  into 
day  spaces,  each  of  which  is  subdivided  into  two-hour  intervals. 
High  drum  record  sheets  are  ruled  for  temperatures  varying 
from  —50°  to  120°;  low  drum  strips  are  usually  ruled  from  o° 
to  100°. 


THE  THERMOGRAPH  191 

High  drum  thermographs  are  used  very  generally  in  meteor- 
ological work.  Low  drum  strips  have  all  the  temperature  range 
necessary  for  greenhouses,  refrigerating  establishments  and 
freight  cars  containing  perishable  goods.  They  are  used  in 
many  Weather  Bureau  stations  where  the  yearly  range  does  not 
materially  exceed  100  degrees.  For  most  stations,  and  for 
general  military  use,  a  high  drum  thermograph  is  advisable. 

Thermographs  are  not  so  accurate  as  standard  thermometers. 
In  very  damp  weather  the  expansion  and  swelling  of  the  paper 
on  which  the  record  is  made  affects  the  accuracy.  Inasmuch  as 
the  paper  rests  on  the  lower  collar  of  the  drum,  the  upward 
expansion  of  the  paper  may  render  the  record  of  the  maximum 
2  or  3  degrees  too  low.  In  any  case,  the  maxima  and  minima 
should  be  compared  with  those  of  the  registering  thermometers, 
and  the  corrections,  plus  or  minus,  noted  on  the  record 
sheet. 

If  the  thermograph  record  does  not  coincide  with  the  ther- 
mometer readings  an  adjustment  screw  will  bring  the  pen  to  the 
proper  position.  It  is  a  good  plan  to  adjust  the  pen  so  that  the 
minimum  coincides  with  that  of  the  minimum  thermometer. 
The  time  of  the  minimum  may  always  be  determined  from  the 
thermograph  sheet,  and  this  is  one  of  its  important  uses.  As  a 
rule,  the  minimum  temperature  occurs  a  short  interval  before 
sunrise.  During  the  progress  of  a  cold  wave  there  may  be  a 
steady  fall  of  temperature  covering  a  period  of  two  days.  Fre- 
quently the  fall  of  temperature  continues  from  12:01  A.M.  to 
11:59  P-M.1  The  "lowest  this  morning"  is  therefore  not  the 
minimum  of  the  day;  and  though  this  fact  may  escape  the 
notice  of  the  observer,  it  will  not  escape  the  record  of  the  thermo- 
graph. 

The  recording  pen  of  the  thermograph  may  lag  anywhere 
from  five  minutes  to  forty-five  minutes  behind  the  actual  tem- 
perature. In  very  damp  weather  the  lag  is  usually  the  greatest, 
and,  in  fixing  the  time  at  which  a  given  temperature  occurred, 
this  fact  must  be  taken  into  the  calculation.  An  observer  who 
studies  the  vagaries  of  his  instruments — and  they  are  many — 
will  learn  how  to  master  them. 

1  These  figures  are  generally  employed  in  weather  bureau  and  in  meteor- 
ological time  to  avoid  the  confusion  that  results  from  the  use  of  the  term 
"midnight." 


192     MEASUREMENT  OF  TEMPERATURE:    THERMOMETERS 

High-air  Thermographs. — High-air  temperature  observa- 
tions are  usually  obtained  by  thermographs  secured  to  kites  or 
balloons.  In  manned  balloons  a  very  sensitive  thermograph 
is  contained  in  a  tube  through  which  a  current  of  air  is  forced. 
This  instrument,  the  Assmann  aspirator,  is  far  more  convenient 
than  an  ordinary  thermograph.  Experience  has  shown  that 
unless  the  air  is  in  rapid  motion,  registration  is  too  slow  to  be 
trustworthy.  A  mechanical  fan  moves  the  air  through  the  tube 
at  the  rate  of  about  12  feet  per  second. 

In  another  form  of  instrument  decreasing  pressure  moves  a 
plate  in  one  direction  while  the  stylus  of  a  bi-metallic  ther- 
mometer records  with  a  motion  at  right  angles  thereto.  A 
clock  is  not  required  in  this  type  of  instrument;  it  is  therefore 
lighter  and  more  convenient. 

The  Black-bulb  Thermometer. — This  instrument,  now  little 
used,  consists  of  a  maximum  standard  thermometer,  the  bulb 
of  which  is  covered  with  a  coat  of  lampblack  and  encased  in  a 
vacuum  tube.  Originally  it  was  designed  for  the  measurement 
of  solar  radiation.  A  thermometer  of  this  sort,  exposed  to  direct 
sunshine,  registers  a  temperature  many  degrees  higher  than  does 
an  ordinary  thermometer,  but  the  degree  varies  according  to  the 
thickness  and  the  quality  of  the  lampblack.  It  is  therefore  a 
very  imperfect  instrument  for  the  purpose  designed. 

The  black-bulb  thermometer  roughly  measures  the  tem- 
perature which  popular  tradition  terms  "sensible"  heat;  exposed 
to  direct  sunshine,  the  temperature  registered  is  from  a  few 
degrees  to  60  or  more  degrees  higher  than  the  temperature 
registered  by  the  ordinary  thermometer.  With  a  high  humidity, 
or  in  smoky,  dusty  or  foggy  air,  the  black-bulb  thermometer 
registers  much  lower  than  in  clean  air. 


CHAPTER  XVI 

THE  MEASUREMENT  OF  PRESSURE:    THE   MERCURY 

BAROMETER 

Two  terms  may  be  used  to  express  the  gravity  of  the  air — 
weight  and  pressure.  The  weight  of  air  is  used  more  properly 
to  express  the  gravimetric  force  of  a  given  volume :  thus,  I  cubic 
foot  of  air  at  normal  pressure  and  temperature  of  32°  F  weighs 
1.29  troy  ounces  (i  cu.  cm.  =  0.0013  gram).  In  meteorology  it 
is  more  convenient  to  consider  the  weight  of  a  column  of  air 
throughout  its  whole  extent,  from  sea  level  upward.  At  sea 
level  such  a  column  of  air  presses  upon  the  surface  with  an 
average  force  of  14.7  pounds,  a  pressure  empirically  termed  I 
atmosphere. 

The  pressure  of  such  a  column  varies,  however,  not  only 
from  day  to  day,  but  from  hour  to  hour.  If  a  mass  or  a  wave 
of  air  accumulates  over  a  given  locality  the  pressure  increases; 
conversely,  if  a  depression,  or  a  trough,  occurs,  the  pressure 
decreases.  As  the  observer  goes  from  sea  level  to  a  higher 
altitude  the  pressure  decreases  also.  At  an  altitude  of  19,000 
feet,  a  height  occasionally  reached  by  airmen,  the  pressure  is 
about  half  of  that  at  sea  level. 

The  pressure  of  the  air  may  be  determined  by  weighing  it; 
it  is  much  more  convenient  to  compare  it  with  a  column  of 
mercury  which  balances  it.  A  balance  constructed  for  this 
purpose  is  a  barometer.  The  theoretical  construction  of  the 
barometer  is  simple;  it  consists  of  a  glass  tube  about  33  inches 
long,  closed  at  one  end,  filled  with  mercury,  and  inverted  with 
the  open  end  in  a  cup  of  mercury.  The  column  of  mercury 
within  the  tube  exactly  balances  a  column  of  air  of  equal  sec- 
tion. If  air  accumulates,  the  increased  pressure  on  the  mercury 
in  the  cup  forces  the  column  higher  in  the  tube;  conversely,  a 
decrease  in  pressure  causes  the  mercury  column  to  shorten; 

193 


194     MEASUREMENT  OF   PRESSURE:    MERCURY  BAROMETER 

)A 


The  mercury  barometer.     Sectional  view  showiner  construction  of  cistern 


CONSTRUCTION  OF  THE  BAROMETER  195 

the  decreased  pressure  cannot  force  so  much  mercury  into  the 
tube. 

Construction  of  the  Barometer. — A  common  form  of  the 
barometer  is  a  siphon  tube,  the  short  arm  of  which  is  enlarged 
to  a  bulb,  and  this  constitutes  the  mercury  cistern.  The  baro- 
metric column  is  the  distance  between  the  upper  and  the  lower 
surface  of  the  mercury. 

The  Weather  Bureau  pattern  is  more  complicated.  The  tube 
has  a  caliber  usually  about  0.25  inch.  The  cistern  or  tank  con- 
taining the  mercury  consists  of  a  short  cylindrical  glass  tube. 
The  cover  is  a  piece  of  boxwood  perforated  to  receive  the 
barometer  tube  and  flanged  to  fit  the  cylindrical  glass  section  of 
the  cistern.  The  lower  part  of  the  cistern  is  a  broad  ring,  of 
boxwood,  flanged  and  fitted  to  the  lower  edge  of  the  glass 
cylinder.  To  the  lower  part  of  the  boxwood  ring  a  kidskin  bag 
is  attached.  The  mercury  fills  the  bag  and  reaches  nearly  to 
the  top  of  the  glass  cylinder.  The  cistern  fits  snugly  into  a 
cylindrical  metal  box.  A  plug  within  the  cylindrical  box, 
operated  by  a  screw  at  the  bottom,  partly  supports  the  bag  of 
mercury.  The  construction  may  be  likened  to  a  glass  tube 
projecting  vertically  from  the  mouth  of  a  rubber  bag  filled  with 
water.  Pressure  on  the  bag  forces  water  up  the  tube;  release 
of  pressure  causes  it  to  lower  in  the  tube. 

The  object  of  the  leather  bag  and  the  plug  is  two-fold; 
it  enables  the  observer  to  raise  or  to  lower  the  mercury  in  the 
cistern  'so  that  the  surface  touches  the  ivory  point  which  is 
the  end  of  the  scale,  thereby  giving  a  more  accurate  reading; 
it  also  enables  the  observer  to  close  the  cistern  and  tube,  so  that 
the  barometer  may  be  carried  in  any  position  without  permitting 
the  mercury  to  escape  from  the  tube,  or  air  to  enter  it. 

The  cistern  of  this  pattern  of  barometer  is  commonly  known 
as  the  Fortin  cistern.  It  was  improved  by  Henry  J.  Green. 
The  improved  pattern  is  used  by  the  Weather  Bureau.  The 
Tuch  cistern,  also  used  by  the  Weather  Bureau,  has  a  piston 
bottom  that  can  be  raised  and  lowered  by  a  thumb-screw. 
The  Weather  Bureau  pattern  of  the  barometer  made  by  the 
Taylor  Instrument  Companies  has  a  similar  device.  In  each, 
a  stopper  pressed  against  the  mouth  of  the  tube  secures  the 
mercury  so  that  the  barometer  may  be  transported  with  a  mini- 
mum of  risk. 


196      MEASUREMENT  OF   PRESSURE:    MERCURY  BAROMETER 

Fixed-cistern  barometers  have  not  been  favorably  considered 
by  meteorologists.  The  objection  to  them  on  the  whole  is  not 
well  founded.  If  the  scale  has  been  compensated  a  fixed-cistern 
barometer  will  meet  all  the  requirements  of  accuracy  demanded 
by  ordinary  meteorological  measurements.  It  is  more  service- 
able for  use  at  sea  than  a  cistern  of  the  Fortin  type.  It  is  less 
likely  to  injury  in  transportation. 

The  necessity  of  a  compensated  scale  may  be  understood 
from  the  following  facts.  The  sectional  area  of  the  cistern  is 
about  fifty  times  the  sectional  area  of  the  tube.  If  the  atmos- 
pheric pressure  increases,  say,  from  29.00  inches  to  30.00  inches 
the  rise  of  I  inch  in  the  tube  is  balanced  by  a  fall  of  0.02  inch 
in  the  cistern.  The  true  height  of  the  column  therefore  is  30.02 
inches.  When  the  sectional  areas  of  both  cistern  and  tube 
have  been  accurately  determined,  an  empiric  scale  compensated 
for  the  instrument  may  be  engraved  to  meet  the  requirements 
of  accuracy.  Should  the  tube  be  broken,  however,  a  new  scale 
will  be  required  inasmuch  as  the  caliber  of  tubes  varies  con- 
siderably. 

A  barometer  with  a  fixed  cistern,  made  by  Schneider 
Brothers,  is  highly  regarded  among  officials  of  the  Weather 
Bureau.  A  feature  of  this  barometer  is  the  facility  with  which 
the  mercury  can  be  made  secure  within  the  cistern  and  tube, 
so  that  the  instrument  will  not  lose  its  adjustment. 

Fixed-cistern  barometers  may  not  meet  the  requirements  of 
precision  measurements  so  well  as  instruments  of  the  Fortin 
type  of  cistern;  but  for  marine  purposes  or  for  field  work,  their 
simplicity  of  construction  and  stability  commend  instruments  of 
this  character.  When  securely  adjusted,  they  may  be  trans- 
ported over  rough  wagon  roads  and  carried  in  any  position 
without  especial  care. 

The  tube  of  the  barometer  is  inclosed  in  a  case  with  the 
necessary  openings  which  permit  the  height  of  the  mercury 
column  to  be  read.  Weather  Bureau  barometers  are  of  the 
"gun-barrel"  type,  tube  and  cistern  being  inclosed  in  a  cyl- 
indrical metal  case.  Openings,  or  "windows"  are  cut  in  the 
sides  so  that  the  top  of  the  column  of  mercury  is  always  in 
sight. 

The  scale  of  Weather  Bureau  barometers  is  on  the  left  side 
of  the  window.  It  is  a  strip  of  white  metal,  with  slotted  screw 


INSTALLATION  OF  THE  BAROMETER  197 

holes  so  that  it  may  be  adjusted  to  compensate  corrections  which 
are  constant,  especially  capillarity.  The  inch  is  divided  into 
tenths  and  subdivided  into  twentieths.  The  scales  of  com- 
mercial barometers  are  usually  without  compensation  adjustments. 

The  vernier  enables  the  observer  to  read  the  height  of  the 
column  to  two  one- thousandths  of  an  inch,  and  to  estimate 
it  to  a  one-thousandth  part.  On  commercial  barometers  the 
vernier  enables  the  observer  to  read  accurately  to  the  one- 
hundredth  part  of  an  inch.  A  rack  and  pinion  moved  by  a 
milled  screw  enable  the  observer  to  adjust  the  vernier  to  the 
height  of  the  mercury. 

The  thermometer  set  into  the  metal  case  of  the  Weather 
Bureau  barometer  is  always  a  standard  instrument,  whether 
carrying  a  certificate  or  not.  The  scale  is  etched  on  the  tube 
to  single  degrees,  but  it  may  be  read  to  half-degrees  in  accord- 
ance with  the  temperature  corrections  which  are  calculated  to 
half-degrees. 

The  Installation  of  the  Barometer. — When  a  barometer  is 
sent  from  the  manufacturer,  or  is  issued  from  the  Weather 
Bureau,  it  is  pretty  certain  to  be  in  good  order  and  ready  for 
installation.  It  is  an  almost  universal  custom  to  wrap  the 
instrument  first  in  tissue  paper,  then  in  cotton  flannel,  and 
finally  in  stout  wrapping  paper.  The  packing  case  should  be 
so  large  that  the  elasticity  of  the  packing  material  will  com- 
pensate any  jar  that  may  occur  from  ordinary  handling.  Ba- 
rometers sent  out  by  the  Weather  Bureaus  are  usually  packed  in 
cases  designed  for  the  purpose.  Ordinary  precaution  suggests 
that  the  cover  of  the  packing  case  should  be  fastened  with 
screws  and  not  with  nails. 

When  a  barometer  is  sent  by  messenger  it  should  be  sent 
either  in  a  leather  case  or  a  box  designed  for  the  purpose,  with 
the  handle  so  placed  that  it  must  be  carried  cistern  uppermost. 
It  should  not  be  allowed  to  rest  with  the  end  on  the  floor  of  a 
moving  vehicle. 

Before  the  barometer  is  removed  from  the  packing  case, 
the  position  most  advantageous  for  it  should  be  determined. 
A  wall  or  partition  that  is  easily  shaken  should  be  avoided. 
A  position  on  a  window  frame  or  near  the  corner  of  a  room  is 
often  the  best  available.  A  position  where  the  temperature  is 
not  subject  to  sudden  change  is  very  desirable. 


198     MEASUREMENT  OF  PRESSURE:    MERCURY  BAROMETER 

Weather  Bureau  barometers  are  provided  with  a  glass- 
paneled  containing  box,  the  front  and  right  side  of  which  swing 
open.  Many  observers  prefer  a  plain  board  mounting.  Marine 
barometers  are  usually  contained  in  a  box  with  an  outrigger 
which  permits  them  to  be  removed  to  a  position  convenient  for 
reading. 

The  box  or  supporting  board  must  be  mounted  so  as  to  be 
vertical  in  all  meridians.  Metal  eyelets,  or  hangers,  accompany 
the  supporting  box  or  board.  When  in  place,  there  should  be 
no  "wiggle"  or  dead  motion.  The  hangers  will  be  found  in  such  a 
position  that  the  barometer  swings  in  the  middle  of  the  lower  ring. 

When  the  barometer  is  removed  from  the  packing  case  it 
should  be  lifted,  cistern  uppermost,  and  laid  on  a  table  or 
bench  to  be  unwrapped.  Until  it  is  finally  in  position  it  should 
be  moved  about  cistern  uppermost.  When  the  wrappings  are 
removed,  it  should  be  carried  cistern  uppermost  to  the  support 
and  turned  carefully  top  end  up.  The  cistern  end  should  be  put 
within  its  supporting  ring  before  it  is  hung  upon  the  hook  of 
the  support.  If  the  box  or  the  supporting  board  has  been 
accurately  leveled,  the  cistern  will  swing  freely  in  the  support- 
ing ring.  The  centering  screws  in  the  ring  may  then  be  turned 
until  each  barely  touches  the  cistern  box.  In  case  the  screws 
are  lost,  pegs  of  soft  wood,  whittled  to  the  right  size,  will  answer 
temporarily.  The  case  of  the  barometer  should  turn  freely  on 
the  swivel,  but  there  should  be  no  dead  motion. 

The  Care  of  the  Barometer. — Except  in  unusual  cases,  a 
mercurial  barometer  should  be  kept  indoors  in  a  position  where 
the  temperature  is  as  nearly  uniform  as  possible.  At  tem- 
peratures materially  below  10°  F  the  readings  of  barometers 
side  by  side  may  vary  enough  to  give  concern  to  a  conscientious 
observer.  When  the  temperature  is  materially  below  zero,  F, 
at  an  altitude  of  5000  feet,  more  or  less,  the  readings  are  often 
of  uncertain  value.  The  moral  is  obvious.  Uniform  and  con- 
stant conditions  are  necessary  for  uniform  results. 

The  compensation  for  capillarity  is  usually  corrected  by 
adjustment  of  the  scale.  Mercury  does  not  "wet"  glass;  there- 
fore the  surface  of  the  tube  not  only  tends  to  retard  the  rise  of 
the  column,  but  prevents  the  mercury  from  assuming  a  level 
surface  at  the  top.  The  rounded  surface  is  the  meniscus,  the 
shape  of  which  changes  from  time  to  time,  as  pressure  varies. 


CARE  OF  THE  BAROMETER  199 

With  a  rising  column  the  convexity  is  visibly  greater  than 
with  a  falling  column.  The  larger  the  bore  of  the  tube  the  less 
the  correction  for  capillarity.  A  tube  with  a  bore  of  less  than 
0.25  inch  should  be  avoided.  Inasmuch  as  the  meniscus  of  the 
larger  tube  has  a  narrower  range,  the  readings  during  changes 
are  a  little  more  accurate  with  a  tube  of  larger  bore. 

When  a  barometer  is  new,  the  surface  of  the  mercury  is 
very  bright.  In  the  course  of  two  or  three  years — or  less — 
the  surface  of  the  mercury  in  the  cistern  may  become  oxidized, 
turning  gray.  Although  unsightly,  this  condition  offers  no 
material  interference  with  accurate  reading.  In  time,  also, 
the  vacuous  part  of  a  poorly  constructed  barometer  may  acquire 
a  gray  tint  owing  to  the  use  of  impure  mercury  in  filling  the 
tube  and  cistern.1  Although  this  may  not  affect  the  reading 
appreciably,  it  is  a  mark  of  careless  workmanship,  and  such  an 
instrument  should  be  sent  to  a  reputable  maker  to  be  refilled 
with  clean  mercury. 

After  a  few  years  of  service  the  film  on  the  surface  of  the 
mercury  may  require  cleaning.  Emptying,  cleaning,  and  refill- 
ing a  barometer  tube  is  a  delicate  task  even  for  trained  experts; 
it  should  not  be  attempted  by  one  without  experience. 

A  clean  room,  free  from  dust,  is  desirable  for  barometers. 
Dust  is  not  preventable,  but  it  should  not  be  permitted  to 
accumulate  on  instruments.  A  soft,  damp — not  wet — cloth 
will  remove  and  gather  it  without  scattering;  a  camel's  hair 
brush  will  remove  it  from  corners  and  crevices  which  the  cloth 
does  not  reach.  If  glass  cylinder  and  tube  are  clean  and 
bright  there  need  be  but  little  error  in  setting  the  mercury  to 
the  scale,  or  in  cutting  the  top  of  the  meniscus  sharply  by  the 
sliding  windows. 

If  a  barometer  is  to  be  removed  from  its  support  the  mercury 
in  the  cistern  should  first  be  raised  until  the  mercury  in  the  tube 
is  flush  with  the  opening  near  the  top  of  the  case.  If  it  is  to  be 

1  Pure  mercury  in  a  saucer  made  chemically  clean  leaves  no  stain  or 
metallic  film  when  shaken  about.  If  it  contains  even  a  trace  of  other  metal 
— lead,  zinc,  or  tin — spots  and  streaks  will  adhere  to  the  saucer.  If  the  mer- 
cury is  not  freshly  distilled  it  may  contain  moisture.  Mercury  may  be  freed 
of  its  own  oxide  by  filtering  through  a  clean  paper  funnel  with  a  pin-hole 
perforation  at  the  bottom.  The  mercury  used  in  Weather  Bureau  barometers 
is  chemically  pure. 


200      MEASUREMENT  OF  PRESSURE:    MERCURY  BAROMETER 

removed  to  a  position  materially  lower,  allowance  should  be 
made  for  the  increase  in  pressure.  Unless  this  precaution  is 
observed,  the  pressure  may  be  great  enough  to  force  mercury 
through  the  joints  of  the  cistern. 

The  following  instructions  concerning  the  removal  from 
position  are  issued  by  the  Weather  Bureau:  "When  moved 
about,  the  cistern  end  should  be  carried  uppermost.  To  turn 
the  barometer  tube-end  up,  bring  it  gradually  to  a  horizontal 
position,  watching  for  a  small  bubble  at  the  cistern.  This 
should  not  be  large,  nor  should  it  be  absent,  in  which  case  there 
may  be  serious  pressure  from  within,  tending  to  force  the 
mercury  out  of  the  cistern.  If  necessary  the  adjusting  screw 
should  be  turned  so  that  the  bubble  is  not  larger  than  the  space 
within  which  a  dime  can  be  placed.  If  there  is  an  air  vent,  as 
in  the  Tuch  cistern,  as  soon  as  the  mercury  is  raised  to  the  top 
of  the  cistern,  close  the  air  vent  tight  and  continue  screwing  up 
the  cistern  until  the  top  of  the  column  reaches  the  summit  of 
the  opening  in  the  metal  tube.  Avoid  raising  the  mercury  in 
the  cistern  until  the  tube  is  entirely  filled  with  mercury.  Do 
not  strain  the  screw  if  it  turns  hard;  mercury  may  have  leaked 
from  the  cistern  and  there  may  not  be  enough  to  fill  the  tube." 
If  no  air  has  entered  the  upper  end  of  the  tube,  when  the 
barometer  is  inclined  about  half  way  the  mercury  will  rise  to 
the  top  of  the  tube  with  a  slight  but  distinct  click;  and  when 
the  instrument  is  nearly  horizontal  a  bubble  should  appear  at 
the  cistern." 

The  foregoing  cautions  apply  to  Fortin  type  barometers 
chiefly,  but  will  apply  in  some  respects  to  other  types.  In  the 
installation,  removal,  and  care  of  other  barometers,  the  direc- 
tions of  the  makers  should  be  followed.  Marine  barometers 
are  provided  with  tubes  a  considerable  portion  of  which  is  con- 
stricted to  prevent  the  ' 'pumping"  of  the  mercury  which  the 
motion  of  the  vessel  would  otherwise  cause.  The  constriction 
prevents  the  vacuous  part  of  the  tube  from  filling  quickly.  The 
barometer  must  be  inclined  gradually,  waiting  until  the  flow 
ceases.  By  the  time  it  is  inclined  about  40  degrees  the  mercury 
will  have  filled  the  tube.  It  can  then  be  inverted  and  moved 
about  in  that  position.  Because  the  cistern  is  partly  filled  only, 
a  marine  barometer  is  easily  put  out  of  adjustment  during  trans- 
portation. 


BAROMETER  SCALES  AND  STANDARDS  201 

When  a  barometer  is  inclined  so  that  the  mercury  is  near 
the  top  of  the  tube,  a  slight  lengthwise  movement  will  cause  it 
to  flow  to  the  top,  striking  it  with  an  audible  "click."  There  is  a 
tradition  that  the  character  of  the  vacuum  can  be  determined 
by  the  character  of  the  sound;  but  inasmuch  as  trained  experts 
are  sometimes  deceived,  the  value  of  the  click  as  a  test  is  uncer- 
tain; and  inasmuch  as  such  a  practise  is  likely  to  break  the 
tube,  the  negative  value  is  pretty  certain. 

Barometer  Scales  and  Standards. — For  many  years  baro- 
metric pressure  was  expressed  in  the  linear  units  of  the  country. 
The  adoption  of  the  metric  system  in  several  states  of  Europe 
changed  the  use  of  local  units  to  metric  units.  The  metric 
system  has  been  authorized  to  be  used  in  the  United  States, 
but  the  use  has  not  been  made  compulsory.  It  is  employed  in 
laboratories  and  in  certain  scientific  work,  but  not  in  the  manu- 
facture of  precision  machinery  unless  definitely  ordered.  It  is 
not  used  for  commercial  purposes  in  English-speaking  countries. 
In  the  latter,  barometric  pressure  is  expressed  in  inches.  Metric 
scale  barometers  are  furnished  on  order  by  the  makers. 

So  far  as  choice  between  the  two  scales  is  concerned  there  is 
not  much  difference.  Each  is  intelligible  in  the  locality  where  it 
is  used.  So  far  as  the  keeping  of  records  is  concerned  there  is 
neither  gain  nor  loss;  each  requires  four  figures  and  a  decimal 
point. 

Physicists  who  use  the  metric  system  of  measurements  find 
it  convenient  to  use  the  dyne — a  force  that  will  impart  to  one 
gram  an  acceleration  in  velocity  of  one  centimeter  per  second — as 
the  unit  of  pressure.  The  pressure  base  proposed  for  barometric 
measurements  is  1,000,000  dynes.  This  value  is  not  sea  level 
pressure,  but  the  average  pressure  at  a  height  of  106  meters 
(348  feet)  above  sea  level.  The  unit  is  the  kilobar,  or  1000 
millibars.  The  conventional  atmosphere  of  29.92  inches  is 
1013.2  millibars. 

To  the  great  majority  of  observers  any  barometer  scale  is 
more  or  less  empiric.  By  long  training  and  habit  one  gradually 
acquires  a  mental  value  of  the  figures  which  express  pressure 
and  these  become  visual  proportions  that  can  be  compared  in 
the  mind.  It  is  difficult  to  change  the  results  of  this  education; 
it  likewise  requires  time.  So  far  as  expression  of  barometric 
terms  of  pressure  are  concerned,  there  is  not  the  slightest  gain 


202      MEASUREMENT  OF   PRESSURE:    MERCURY  BAROMETER 

in  the  substitution  of  the  metric  for  the  inch  scale  or  vice  versa.1 
When  records  have  covered  considerable  periods  of  time  a  change 
of  either  to  the  other  results  not  only  in  confusion  but  in  positive 
loss. 

Barometer  Observations  and  Records. — Weather  Bureau 
barometer  records  are  made  at  8  :oo  A.M.  and  8  :oo  P.M.  Observers 
usually  note  any  changes  that  may  have  occurred  during  the 
day.  Making  an  observation  for  record  that  shall  meet  the 
demands  for  reasonable  accuracy  requires  a  certain  amount  of 
experience  and  familiarity  with  the  barometer. 

Inasmuch  as  temperature,  pressure,  humidity  and  wind 
observations  are  to  be  taken  at  clock  time,  and  all  these  require 
about  ten  minutes  in  the  aggregate,  it  is  sometimes  necessary 
to  decide  quickly  as  to  preference  of  order.  During  heavy 
storms  a  variation  in  barometric  pressure  may  change  visibly; 
and  in  winter  weather,  temperature  may  rise  more  than  I  degree 
between  8:00  A.M.  and  8:10  A.M.  Judgment  and  experience 
must  determine.  As  a  rule,  however,  two  minutes  will  be  a 
generous  allowance  of  time  for  temperature  observation.  To 
make  such  observations  habitually  out  of  the  established  time 
should  be  a  good  reason  for  looking  with  suspicion  upon  the 
records  thus  made;  if  for  any  reason  an  observation  is  made  out 
of  time,  the  fact  and  the  time  should  be  noted.  Slipshod  prac- 
tise in  the  time  of  making  observations  may  not  impair  the 
results,  but  they  certainly  impair  the  character  of  the  observer. 

Because  body  warmth  may  affect  the  attached  thermometer, 
the  temperature  should  first  be  noted.  It  is  best  to  record  the 
temperature  to  the  nearest  half-degree.  In  field  work,  especially, 
if  the  temperature  is  within  a  few  degrees  of  the  freezing  point, 
reading  the  temperature  to  the  nearest  degree  will  be  sufficient 
for  ordinary  determination.  Below  29°  F  the  temperature 
corrections  are  additive;  above  28°  F  they  are  subtractive. 

The  milled  screw  at  the  bottom  of  the  case  raises  or  lowers 
the  mercury  to  the  scale.  When  the  surface  of  the  mercury 

1  If  a  change  from  the  English  mercury-inch  system  should  become  de- 
sirable, the  millibar  scale  would  be  considered  preferable  to  any  other  so  far 
proposed.  At  Greenwich,  where  the  acceleration  is  981.17  centimeters,  the 
standard  of  pressure  is  1,013,800  dynes;  at  Paris  it  is  1,013,600  dynes;  in 
the  United  States  (U.  S.  Coast  Survey  determination  for  Lat.  45°)  the 
standard  is  1,013,200  dynes,  acceleration  980.62  centimeters  or  32.16  feet. 


OBSERVATIONS  AND  RECORDS  203 

in  the  cistern  touches  the  ivory  point  it  is  at  the  zero  of  the 
scale  and  the  distance  to  the  top  of  the  column  is  the  observed 


Adjusting  the  surface  of  the  mercury  to  the  ivory  point  is 
best  accomplished  in  many  instances  by  the  use  of  artificial 
light.  Where  convenient  an  extension  socket  to  the  nearest 
light  plug  is  the  best  method;  a  flashlight  will  answer  all  pur- 
poses. There  are  several  ways  to  determine  tangency  of  the 
ivory  point  and  the  mercury: 

Contact  between  the  point  and  its  shadow  on  the  surface 
of  the  mercury. 

Making  a  visible  dent  in  the  mercury  with  the  ivory  point; 
then  lowering  the  surface  until  the  dent  disappears. 

With  the  eye  in  the  horizontal  plane  of  the  end  of  the  ivory 
point,  noting  the  position  when  the  light  space  between  the 
point  and  the  mercury  ceases  to  appear. 

Observers  usually  prefer  the  last  method.  In  practise, 
the  first  method  is  associated  with  it.  The  second  method  is 
fairly  safe  when  the  surface  of  the  mercury  is  bright,  but  it  is 
not  easy  to  discern  the  dent  if  the  mercury  is  tarnished.  Experi- 
ence will  usually  determine  the  method  by  which  the  observer 
will  obtain  the  most  accurate  results. 

Setting  the  vernier  scale  exactly  to  the  meniscus,  or  rounded 
top  of  the  mercury  in  the  tube,  is  not  always  easy.  The  first 
requisite  is  a  clean  tube.  The  film  that  gathers  upon  the  cut- 
side  of  the  tube  in  damp  weather  catches  dust  and  interferes 
with  the  transparency  of  the  glass.  The  moral  is  obvious: 
the  tube  should  be  clean.  The  refraction  of  the  glass  has  a 
tendency  to  produce  a  "drop,"  making  it  slightly  difficult  to 
adjust  the  two  edges  of  the  vernier  shutter  so  that  the  line  of 
sight  is  precisely  tangent  to  the  meniscus.  The  eye,  of  course, 
must  be  in  a  line  with  the  edges  of  the  windows. 

A  very  great  part  of  the  value  of  barometer  observations  con- 
sists of  the  knowledge  that  may  be  obtained  by  comparisons. 
In  order  to  compare  observations  they  must  be  reduced  to  a  com- 
mon base;  namely,  a  temperature  of  32°  F  and  sea  level.  The 
temperature  correction,  except  as  noted,  is  subtractive;  the 
altitude  correction  is  additive,  except  as  the  station  may  be 
below  sea  level.  Death  Valley  and  Imperial  Valley,  California, 
are  stations  in  the  United  States  to  which  this  exception  applies. 


204     MEASUREMENT  OF   PRESSURE:    MERCURY  BAROMETER 

A  correction  for  latitude  is  required  at  Weather  Bureau 
stations.  This  correction  in  the  United  States,  Alaska  excepted, 
varies  from  nothing  at  Lat.  45°  to  0.05  inch  in  the  southern 
part  of  the  country.  It  is  additive  in  latitudes  higher  than 
45°  and  subtractive  in  latitudes  lower.  Being  a  constant,  it 
may  be  included  in  the  sea  level  reduction. 

Except  for  weather  bureau  records,  or  for  comparison  with 
sea  level  records,  reduction  to  sea  level  is  not  necessary.  In 
general,  station-altitude  readings  and  the  oscillations  in  pressure 
are  of  greater  value  to  the  observer  than  reduced  readings. 
This  is  notably  the  case  in  aviation.  It  is  often  necessary  to 
know  whether  one  is  entering  a  region  of  increasing  or  of  decreas- 
ing pressure.  The  difference  involves  not  only  questions  of  plane 
support;  it  is  also  the  distinction  between  clearness  and  cloudi- 
ness. 

Obtaining  Station  Altitudes. — The  altitude  of  a  permanent 
station  should  be  determined  as  closely  as  is  possible  with  ordi- 
nary facilities.  Two  points,  a  "plane  of  reference"  and  a  station 
fixed  point  are  required.  The  first  should  be,  if  possible,  a 
bench  mark  of  the  United  States  Coast  Survey,  the  Lake  Sur- 
vey, the  Mississippi  River  Commission,  the  Engineer  Corps,  or 
the  United  States  Geological  Survey.  Of  less  precision  are 
railway  levels  and  city  bench  marks,  and  other  surveys  made 
by  engineers ;  they  will  be  found  useful  for  reference  even  when 
their  precision  is  doubtful.  Railway  station  levels  are  reason- 
ably precise.  The  top  of  the  rail  at  a  designated  point  within 
jyard  limits  may  be  taken  as  a  plane  of  reference. 

If  a  precisely  determined  elevation  is  required  it  can  be 
obtained  best  by  a  survey  from  the  most  accessible  established 
bench  mark.  The  station  fixed  point  should  be  made  on  some 
object  that  is  both  fixed  and  durable.  A  young  and  rapidly 
growing  tree  is  not  a  desirable  object  for  a  station  mark; 
but  a  mark  made  on  a  full-grown  tree  is  not  subject  to  material 
change.  A  stone  post  set  firmly  in  the  ground,  or  a  piece  of 
painted  scantling  attached  firmly  to  the  corner  of  a  building 
will  answer  the  purpose.  The  mark  should  be  of  such  a  charac- 
ter that  it  will  resist  ordinary  weathering.  The  final  point  in  the 
determination  is  the  station  barometer,  that  is,  the  chain  of 
determinations  which  begin  at  an  established  plane  of  reference 
and  end  with  the  ivory  point  within  the  cistern  of  the  barometer. 


ALTITUDES  BY  COMPARATIVE  OBSERVATIONS          205 

The  term  "sea  level"  is  differently  interpreted  in  different 
localities,  "Mean  tide,"  "mean  low  tide,"  "mean  high  tide,"  and 
"mean  sea  level"  are  used.  If  the  local  usage  does  not  conform 
to  that  of  established  Federal  usage,  the  nearest  established 
Plane  of  Reference l  practicable  should  be  sought  a?  a  start- 
ing point.  The  Weather  Bureau  has  established  a  specific 
elevation  for  each  of  its  stations;  the  nearest  station  practi- 
cable therefore  may  be  taken  as  an  initial  point. 

Altitudes  by  Comparative  Barometric  Observations. — 
Reasonably  correct  altitudes  may  be  established  by  synchro- 
nous observations,  one  series  at  an  established  altitude,  the  other 
at  the  place  whose  altitude  is  to  be  determined.  For  this  pur- 
pose the  position  of  known  altitude  should  be  a  Weather  Bureau 
station  or  an  observatory  having  a  standard  barometer  and  an 
observer  of  experience.  If  a  mercurial  barometer  is  used  at 
the  location  whose  altitude  is  to  be  determined,  it  should  be 
allowed  to  "rest" — that  is,  to  adjust  itself  to  the  altitude — for 
a  few  days,  if  possible. 

The  readings  may  be  made  hourly  at  the  same  time  at  both 
stations,  the  height  of  the  mercury,  time  and  temperature  cor- 
rection being  noted.  This  may  be  repeated  for  several  days 
until  the  reduced  readings  are  constant.  If  the  stations  are 
not  far  apart,  a  single  series  of  observations  may  suffice;  if  they 
are  more  than  25  miles  apart  differences  in  pressure  other  than 
those  due  to  altitude  may  interfere. 

For  instance,  six  consecutive  observations  between  the 
stations  show  a  constant  difference,  and  the  lower  pressure  at 
the  upper  station  may  be  assumed  as  a  difference  in  pressure  due 
to  altitude. 

1  Planes  of  Reference  established  by  the  U.  S.  Geological  Survey  are 
established  with  reference  to  mean  sea  level. 


CHAPTER  XVII 

THE   MEASUREMENT   OF   PRESSURE:     THE   ANEROID 

BAROMETER 

The  aneroid  1  barometer  has  become  an  instrument  of  the 
greatest  usefulness  to  the  explorer,  the  meteorologist  and  the 
airman.  To  the  last  named  it  is  indispensable;  no  other  form 
of  barometer  to  take  its  place  has  been  devised.  Its  great 
virtue  is  its  portability. 

The  Construction  of  the  Aneroid  Barometer. — The  essential 
part  of  the  aneroid  barometer  is  the  shallow  metal  box  with 
thin  corrugated  top  and  bottom,  usually  of  German  silver, 
having  a  thickness  of  0.004  inch.  The  corrugation  gives  a 
much  greater  degree  of  expansion  and  contraction  than  would 
be  possessed  by  a  plane  surface. 

The  box  is  the  vacuum  chamber  or  cell.  The  top  and  bottom 
are  so  elastic  that,  when  the  air  is  exhausted,  they  collapse 
almost  completely.  To  prevent  permanent  warping  a  stout 
steel  spring  attached  to  a  stud  in  the  top  of  the  vacuum  box 
pulls  it  into  a  normal  position.  This  mechanism  results  in  a 
surface  that  is  very  sensitive  to  atmospheric  pressure. 

A  train  of  levers,  a  chain  and  a  drum  translate  the  move- 
ments of  the  vacuum  chamber  covers,  caused  by  changing  pres- 
sure, into  a  circular  motion;  an  index  pointer  moves  back  and 
forth  over  an  arc  graduated  to  represent  inches  of  mercury. 
A  movable  scale  with  a  zero  point  that  can  be  set  at  a  desired 
position  encircles  the  barometric  pressure  scale.  This  scale  is 
graduated  to  express  altitudes. 

The  steel  service  spring  and  the  metal  of  the  vacuum  chamber 
are  weakened  by  warmth,  thereby  impairing  the  accuracy  of 
the  readings.  In  some  aneroid  barometers  this  is  offset  to  a 
considerable  degree  by  the  admission  of  a  small  portion  of  dry 

1  The  name  is  not  connected  with  the  Greek  word  meaning  "air";  it  is 
formed  of  two  Greek  words  meaning  "without  a  liquid." 

206 


ADJUSTMENT  OF  ANEROID  BAROMETERS  207 

air  into  the  vacuum  chamber;  in  others,  the  long  lever  extending 
from  the  steel  service  spring  is  made  of  two  strips — brass  and 
steel — brazed  together.  The  altered  length  of  the  lever  is  made 
to  offset  the  weakening  of  the  service  spring.  An  aneroid  of 
this  construction  is  thereby  compensated  for  temperature.  A 
barometer  bearing  the  name  of  the  maker,  and  not  a  fictitious 
name,  is  pretty  apt  to  be  as  it  is  represented.  Among  com- 
mercial aneroid  barometers  one  may  find  instruments  thus 
marked  in  which  the  compensation  is  far  from  perfect. 

When  the  aneroid  is  to  be  used  wholly  to  indicate  weather 
conditions,  compensation,  although  desirable,  is  not  essential. 
On  the  other  hand,  if  it  is  to  be  used  mainly  for  measuring 
altitudes,  compensation  must  be  regulated  with  extreme  care. 
A  compensated  aneroid  requires  no  temperature  correction; 
the  compensation  is  for  the  purpose  of  eliminating  such  correc- 
tions. 

The  Goldschmidt  type  of  aneroid  differs  from  the  type 
commonly  known  by  dispensing  with  the  train  of  levers.  A 
micrometer  screw  working  in  the  cover  of  the  box  measures  on 
its  graduated  rim  the  amount  of  the  movement  of  the  cor- 
rugated top  of  the  vacuum  chamber.  When  operated  by  an 
expert  trained  to  the  use  of  micrometry,  this  type  of  aneroid  . 
possesses  many  merits.  It  is  not  well  adapted  to  general  use. 

At  the  best,  the  aneroid  is  a  delicate  instrument  requiring 
great  care,  especially  if  it  is  a  part  of  an  engineer's  equipment. 
The  reading  of  an  instrument  having  a  large  dial  will  change 
with  any  material  change  in  its  position.  If  the  user  watches 
its  variations,  however,  such  erratic  changes — and  they  are 
small — will  not  result  in  erroneous  readings. 

Adjustment  of  Aneroid  Barometers. — An  aneroid  barometer 
is  sometimes  blamed  because  it  does  not  agree  with  the  reading 
of  a  mercurial  barometer  at  the  same  level.  This  may  occur 
when  rapid  changes  in  pressure  are  taking  place.  An  aneroid 
of  the  best  type  is  very  sensitive.  It  responds  to  changes  in 
pressure  far  more  quickly  than  does  a  mercurial  barometer. 
Because  of  its  complex  mechanism  it  is  easily  put  out  of  adjust- 
ment; moreover,  it  will  get  out  of  adjustment  for  causes  that 
are  not  well  known. 

Adjustment  to  a  correct  reading  should  be  made,  if  possible, 
when  pressure  is  stationary.  The  adjustment  is  made  by  means 


208     MEASUREMENT    OF    PRESSURE:     ANEROID    BAROMETER 

of  the  small  screw  in  the  back  of  the  case — usually  the  only 
screw-head  in  sight.  The  index  hand  moves  in  the  same  direction 
that  the  screw-driver  turns.  The  adjusting  should  not  be  used 
to  set  the  index  more  than  three-tenths  of  an  inch.  If  the  error 
is  materially  greater  than  this,  it  is  better  to  have  the  adjust- 
ment made  by  an  expert.  When  this  cannot  be  done  without 
delay,  the  error  may  be  temporarily  reduced,  for  convenience 
in  reading,  to  even  tenths  of  an  inch.  If  it  is  desirable  to  have 
the  index  read  to  sea  level  reduction,  it  may  be  lifted  off  the  stud 
and  replaced  as  nearly  as  possible  in  the  correct  position.  Any 
slight  difference  may  then  be  taken  up  by  the  adjusting  screw. 

In  moving  the  adjustment  screw,  one  must  talce  into  con- 
sideration the  position  in  which  the  instrument  habitually  rests. 
If  a  barometer  which  has  been  adjusted  to  a  hanging  position 
is  laid  upon  its  back,  the  reading  changes  several  hundredths  of 
an  inch,  and  vice  versa.  It  therefore  must  be  held  in  its  habitual 
position  when  the  adjustment  is  made. 

If  the  error  in  reading  is  two-  or  three-tenths,  the  observer 
must  watch  the  readings  for  several  days  to  ascertain  if  the 
adjustment  has  changed.  If  the  index  has  been  set  forward  it 
is  apt  to  "  creep  "  forward  still  further;  if  backward,  the  creep- 
ing will  be  backward.  The  reason  therefor  is  not  known  with 
certainty.  An  aneroid  taken  to  a  higher  elevation  is  apt  to 
respond  quickly  to  the  reduced  pressure;  taken  to  a  level  materi- 
ally lower,  it  may  not  respond  so  quickly.1 

If  an  aneroid  is  in  a  proper  condition,  tapping  the  case  with 
the  finger  will  cause  an  instantaneous  vibratory  movement  of 
the  index  which  will  settle  each  time  to  the  same  position.  If 
it  fails  to  recover  its  normal  position,  bring  the  instrument 
rather  sharply  down  upon  a  chair  cushion  or  cane  seat;  if  the 
index  fails  to  recover  its  position,  or  does  not  vibrate,  a  binding 
at  the  lever  joints  exists,  and  the  instrument  should  go  to  the 
repair  shop.  The  best  test  as  to  whether  or  not  it  is  in  good 

1  An  aneroid  taken  by  the  author  from  sea  level  to  stations  in  Colorado 
varying  from  8000  to  13,000  feet,  responded  promptly  to  the  decrease  in 
pressure  on  the  outward  trip.  After  it  had  been  brought  back  to  sea  level, 
it  registered  an  altitude  of  about  2000  feet.  At  the  end  of  three  weeks  it 
still  varied  by  nearly  0.3  inch  from  normal  pressure.  It  was  therefore  sent 
to  the  manufacturer  to  be  put  in  order.  This  illustration  will  apply  in  many 
instances;  it  does  not  apply  to  aneroids  of  the  better  class  made  at  the  present 
time. 


ENGINEERS'  ANEROID  BAROMETERS  209 

working  condition  is  its  sensitiveness  to  slight  changes  in  eleva- 
tion^-for  instance,  the  difference  in  elevation  between  the  adja- 
cent floors  of  a  house.  The  response  of  the  index  should  be 
instantaneous.  An  aneroid  of  the  best  type  should  show  the 
difference  of  elevation  between  the  top  of  a  table  and  the  floor. 

Engineers'  Aneroid  Barometers. — Within  the  past  few  years 
material  improvements  in  the  construction  of  aneroids  have 
removed  the  defects  noted  in  the  preceding  paragraphs.  On 
ordinary  aneroids  the  divisions  of  the  pressure  scale  are  equal, 
while  those  of  the  altitude  circle  gradually  diminish.  It  is 
evident  therefore  that  a  vernier  could  not  be  used  on  such  a 
scale. 

In  the  scale  graduation  of  the  engineer's  type  of  aneroid, 
the  arrangement  is  reversed;  the  pressure  scale  divisions 
diminish  while  those  of  the  altitude  circle  are  made  equal.  If 
the  scale  divisions  represent  20  feet,  the  vernier  subdivides 
them  into  2-foot  divisions.  Many  of  the  newer  instruments 
have  these  values  in  scale  construction;  on  others  the  scale 
divisions  are  50  feet  and  5  feet. 

Although  the  engineer's  aneroid  is  compensated  for  tempera- 
ture a  slight  temperature  correction  is  advisable  where  the 
difference  in  altitudes  is  considerable.  P.  R.  Jameson  has 
deduced  the  following  rule:  If  the  sum  of  the  number  of  degrees 
at  the  two  stations  is  greater  than  100  degrees  F  (55  degrees  C), 
increase  the  height  by  one  one-thousandth  part  for  each  degree  F 
in  excess  of  100  degrees  F;  if  the  sum  of  the  number  of  degrees 
is  less  than  100  degrees  F,  diminish  the  altitude  by  one  one- 
thousandth  part  for  each  degree  F. 

In  using  the  engineer's  aneroid  for  determining  altitudes  the 
zero  point  may  be  set  at  the  station  of  known  altitude.  For 
reasons  explained  in  a  preceding  paragraph',  such  a  proceeding 
will  not  do  with  an  aneroid  whose  scale  divisions  on  the  altitude 
circle  are  unequal.  In  using  such  an  instrument  the  zero  point 
should  be  set  at  a  designated  position  and  the  correction  made 
for  the  variation  which  the  reading  reduced  to  sea  level  shows 
to  be  necessary. 

Pocket  Aneroid  Barometers. — This  term  is  applied  to  small 
instruments  about  2  inches  in  diameter.  In  quality  they  vary 
from  good  to  poor.  The  chief  virtue  about  them  is  convenience 
and  portability.  In  spite  of  the  name,  the  pocket  is  not  a 


210     MEASUREMENT    OF    PRESSURE:     ANEROID    BAROMETER 

suitable  receptacle  in  which  to  carry  such  an  instrument.  The 
moisture  from  the  body  sooner  or  later  affects  the  metal  mech- 
anism; moreover,  the  knocking  and  banging  which  it  is  likely  to 
receive  if  carried  in  an  overcoat  pocket  are  equally  hurtful.  In 
traveling,  it  is  carried  most  safely  in  a  grip  sack. 

A  very  good  place  for  the  pocket  aneroid  is  the  observer's 
desk  and  it  is  advisable  to  set  the  zero  mark  of  the  altitude 
scale  daily  at  the  index  position.  Thereby  is  inculcated  a  habit 
of  watching  the  barometric  changes  far  more  closely  than  is 
the  custom  when  one  must  go  across  the  room  to  set  the  mer- 
curial barometer  for  a  reading.  The  close  observer  gets  a  much 
better  view-point  of  daily  variations  than  does  the  casual 
observer. 

Specific  Uses  of  the  Aneroid  Barometer. — A  custom  that  is 
wellnigh  universal  makes  the  mercurial  barometer  the  standard 
instrument  for  the  measurement  of  pressure  at  Weather  Bureau 
stations.  There  is  no  doubt  of  the  wisdom  of  this  practise.  For 
a  substantial  instrument  not  easily  getting  out  of  order,  and 
susceptible  of  close  reading,  no  other  form  of  barometer 
approaches  it.  Such  instruments  as  the  Marvin  normal  baro- 
graph represent  the  highest  skill  in  precision  instruments. 

The  modern  aneroid  is  quite  as  essential  as  the  mercury 
barometer  in  the  equipment  of  a  Weather  Bureau  station,  a 
maritime  observatory,  or  a  meteorological  laboratory.  For  use 
on  vessels  it  has  many  advantages  over  the  mercurial  barometer, 
Extras  may  be  carried  on  various  parts  of  the  ship  where  con- 
venience suggests.  Not  the  least  virtue  of  the  ship's  aneroid  is 
the  fact  that  it  responds  to  pressure  changes  more  quickly  than 
does  the  mercurial  barometer. 

Aneroid  Recording  Barometers. — Recording  aneroids  are 
sold  under  various'  copyrighted  names;  in  Weather  Bureau 
cant  such  an  instrument  is  known  as  a  barograph.  The  essential 
feature  is  one  or  more  vacuum  chambers,  a  drum  moved  by  a 
clock,  and  a  ruled  sheet  of  paper  on  which  the  record  is  made. 
The  better  type  of  barograph  has  a  battery  of  eight  vacuum 
chambers,  one  upon  the  top  of  another.  This  arrangement 
permits  the  movement  due  to  pressure  to  be  multiplied  eight- 
fold. A  finer  adjustment  and  a  more  accurate  record  is  gained 
thereby.  Temperature  compensation  is  effected  by  the  admis- 
sion of  a  measured  quantity  of  dry  air  into  one  of  the  chambers. 


RECORDING  ANEROID  BAROMETERS 


211 


The  long  lever,  the  pen  arm,  carries  a  pen  which  presses 
lightly  on  the  ruled  paper  wrapped  around  the  drum.  A  milled 
screw  adjusts  the  pressure  of  the  pen  on  the  paper,  and  a  switch 
enables  the  observer  to  throw  the  pen  off  or  on  at  pleasure.  The 
drum  makes  one  revolution  per  week.  The  record  sheets  are 
ruled  with  horizontal  lines  representing  inches,  halves,  and 
twentieths.  Arcs  of  circles,  having  radii  equal  to  the  length  of 
the  pen  arm,  divide  the  record  sheet  into  midday  and  midnight 
periods,  and  two-hour  intervals.  Eight  o'clock  Monday  morn- 
ing is  the  normal  time  for  changing  the  record  sheets  used  by 
most  observers.  These  contain  day  and  month  spaces  for  record- 


Barograph, 

ing  dates.  In  Weather  Bureau  practise  the  sheets  are  changed 
on  the  1st,  8th,  I5th,  22d  and  29th  days  of  the  month. 

The  clocks  used  in  most  barographs  are  watch  movements 
of  the  finest  type.  It  is  desirable  to  have  the  clock  oiled  and 
cleaned  once  a  year.  The  clock  is  practically  dust-proof,  being 
within  the  drum,  and  protected  also  by  the  glass  case  which 
incloses  the  mechanism  of  the  barograph. 

In  replacing  the  record  sheet  the  observer  must  look  care- 
fully to  two  things;  the  lower  edge  of  the  record  sheet  must 
fit  closely  to  the  collar  at  the  lower  edge  of  the  drum;  the  lines 
of  equal  pressure  must  match  precisely.  A  failure  to  conform 
to  these  requirements  leads  to  incorrect  records. 


212     MEASUREMENT    OF    PRESSURE:     ANEROID    BAROMETER 

The  ink  used  consists  usually  of  glycerine,  water  and  color 
pigment.  If  a  permanent  record  is  required  black  ink  is  prefer- 
able; green  or  blue  is  fairly  permanent;  red  and  purple  fade  in 
the  course  of  a'few  years.  A  red  ink  with  a  madder  or  a  genuine 
carmine  base  will  not  fade.  Aniline  red  inks  are  marketed  as 
"  carmine,"  however,  and  they  are  not  at  all  permanent. 

The  pen  of  the  barograph  has  a  sleeve  which  slips  over  the 
end  of  the  pen  arm.  It  is  not  always  easy  to  remove  it  when 
the  pen  requires  cleaning.  If  it  cannot  be  removed  from  the 
pen  arm,  lift  the  drum  off  the  spindle;  hold  the  pen  firmly 
and  clean  with  a  small  camel's  hair  brush  and  water.  Before 
filling  the  pen,  draw  a  narrow  strip  of  paper  or  a  very  thin 
spatula  blade  through  the  prongs  of  the  pen  in  order  that  the 
ink  may  flow  freely.  With  reasonable  care  for  its  cleanliness 
the  pen  will  make  a  sharply-cut  line;  a  foul  pen  leaves  its  own 
record. 

The  milled  screw  in  the  bar  directly  over  the  vacuum  cham- 
ber adjusts  the  pen  so  that  it  has  the  correct  position  on  the 
record  sheet.  The  pen  may  be  adjusted  to  record  sea  level 
pressure;  but,  as  a  rule,  it  is  better  to  keep  local  pressure. 
The  record  sheets  are  usually  lithographed  with  figures  ranging 
from  28  to  31  inches.  They  are  lithographed  for  other  altitudes 
and  also  without  any  altitude  marks.  A  "  long-range  "  baro- 
graph, registering  from  25  to  31  inches,  is  also  made.  Metric 
charts  may  be  obtained  from  dealers  in  meteorological  sup- 
plies. 

When  a  barograph  is  to  be  moved — if  carried  otherwise 
than  by  hand — the  drum  and  the  ink  bottle  should  be  removed 
and  packed  separately.  The  pen  arm  should  be  fastened  loosely 
to  the  switch  rod.  A  dozen  thicknesses  of  tissue  paper  or  of 
soft  cloth  should  be  wrapped  around  the  glass  case  and  stand; 
they  should  be  fastened  so  firmly  that  the  parts  cannot  jostle. 
With  an  additional  protective  wrapping  of  heavy  paper  the 
instrument  will  ride  safely  in  the  packing  case.  There  should 
be  no  packing  of  any  sort  around  the  vacuum  chambers  and 
levers. 

If  the  barograph  is  carried  by  hand,  the  drum  need  not  be 
removed  from  the  spindle,  but  the  pen  arm  should  be  thrown 
from  the  drum.  A  handle  should  be  fixed  to  the  package  so 
that  it  may  be  carried  in  its  proper  position. 


INTERPRETATION  OF  BAROGRAPH  RECORDS  213 

Interpretation  of  Barograph  Records. — The  usual  eight 
o'clock  observations  of  pressure  furnish  nothing  more  than 
changes  in  pressure  at  twelve-hour  intervals.  For  the  purpose 
of  scientific  study  the  continuous  barogram  is  the  best  object 
lesson.  The  West  India  hurricane,  the  ordinary  storm,  the 
heavy  downpour,  the  thunder-storm  and  the  cold  wave — each 
leaves  its  individual  earmarks  on  the  record  sheet.  In  no  other 
way  can  the  observer  work  out  his  position  and  time  of  diurnal 
inequality  so  well  as  by  the  use  of  the  record  sheet. 

A  discussion  of  the  specific  features  made  by  the  barograph 
pen,  noted  in  the  preceding  paragraph,  is  not  necessary.  Each 
should  be  noted  on  the  record  sheets,  at  the  time  of  its  occurrence. 
At  times  the  specific  features  in  a  record  sheet  enable  an  observer 
to  make  forecasts  that  would  escape  notice  if  the  observations 
were  made  with  an  ordinary  barometer.  Thus,  if  the  trace  of 
the  pen  in  the  progress  of  a  falling  barometer  is  convex,  an 
increasing  violence  of  an  approaching  storm  is  indicated.  In 
the  same  manner,  a  concave  trace  of  the  pen  with  a  rising  ba- 
rometer indicates  an  increasing  force  of  the  wind — frequently 
the  onset  of  a  cold  wave. 

A  close  study  of  the  trace  of  the  barograph  pen  will  enable 
an  observer  to  discern  conditions,  leading  to  fairly  certain  pre- 
dictions, which  otherwise  would  pass  unnoticed.  Continuity 
of  record  constitutes,  to  a  great  degree,  the  value  of  pressure 
records,  and  while  the  records  of  hourly  observations  intelli- 
gently graphed,  might  lead  to  similar  conclusions,  few  meteor- 
ological laboratories  are  so  equipped  as  to  make  such  observa- 
tions possible.  But  the  barograph  catches  and  records  minute 
pressure  alterations  that  might  escape  the  notice  of  the  most 
careful  observers  even  if  hourly  observations  were  recorded. 

There  is  not  much  difficulty  in  comparing  the  graphs  of 
metric  charts  with  those  of  inch  charts.  A  base  line  for  either 
may  be  drawn  on  the  other,  and  distances  or  ordinates  are 
readily  measured  with  dividers  or  with  a  graduated  scale.  A 
record  sheet  ruled  on  tracing  cloth  affords  a  convenient  method 
of  comparison.  The  best  method  of  comparison  is  the  one  which 
best  suits  the  observer. 


CHAPTER  XVIII 
THE  MEASUREMENT  OF  HUMIDITY:    HYGROMETERS 

The  Water-vapor  Content  of  the  Air. — Water  vapor  exists 
in  the  troposphere,  or  lower  shell  of  the  air,  at  all  times,  the 
amount  depending  very  largely  on  the  temperature  of  the  air. 
To  the  best  of  knowledge,  little  if  any  water  vapor  exists  in 
the  air  of  the  stratosphere.  The  high  cirrus  clouds  presumably 
mark  the  upper  limit  of  the  condensation  of  the  water  vapor  of 
the  air. 

The  maximum  proportion  of  vapor — that  is,  the  maximum 
quantity  per  unit  of  volume — depends  on  temperature;  it  is 
independent  of  the  other  constituents  of  the  air.  Were  there 
no  other  constitutents,  the  atmosphere  would  be  an  atmosphere 
of  water  vapor,  and  the  amount  per  unit  of  volume  would  be 
about  the  same  as  under  existing  conditions.  It  is  best,  there- 
fore, to  consider  the  water  vapor  content  as  an  independent 
factor  so  far  as  measurements  are  concerned. 

When  the  air — or  rather,  the  water  vapor  itself— is  near  the 
point  of  saturation  it  is  moist  to  the  senses.  Hygroscopic  sub- 
stances, such  as  sugar,  salt  and  many  other  substances,  absorb 
moisture;  sized  paper  and  starched  fabrics  swell  and  become 
limp.  These  conditions  begin  to  be  noticeable  when  the  water 
vapor  of  the  air  passes  85  per  cent  of  the  amount  that  may 
exist. 

When  the  vapor  content  is  30  per  cent,  or  less,  of  the  amount 
required  for  saturation,  the  dryness  becomes  apparent  to  the 
senses,  especially  to  the  lips  and  throat.  The  gummed  surface 
of  stamps,  labels  and  adhesives  shrinks,  causing  the  paper  to 
curl.  Doors  warp  and  thin  panels  of  wood  shrink  and  split. 

The  moisture  sensation  of  the  air  is  its  humidity.  Before 
saturation  is  reached,  the  moisture  is  in  the  form  of  vapor; 
at  the  point  of  saturation  it  may  appear  in  the  air  as  fog,  or 
cloud;  when  the  ground  temperature  reaches  the  point  of 

214 


MEASUREMENT  OF  HUMIDITY  215 

saturation  condensation  takes  place  in  the  form  of  dew;  or,  if 
below  32°  F  (o°  C)  in  the  form  of  frost. 

The  table,  p.  280,  shows  the  amount  of  moisture  at  different 
temperatures  which  may  be  present  mingled  with  the  air. 
Thus,  at  30°  F  a  little  less  than  2  grains  per  cubic  foot  can  be 
present  before  condensation  begins;  while  at  70°  F  there  may 
be  nearly  8  grains.  In  other  words,  if  2  grains  per  cubic  foot 
were  present  when  the  temperature  was  30°  F,  condensation 
would  be  taking  place;  while,  if  the  temperature  were  70°, 
the  air  would  be  very  dry,  because  only  one-quarter  of  the 
moisture  required  for  saturation  is  present.  Sensible  moisture, 
therefore,  is  relative,  requiring  measurement  of  temperature 
and  absolute  humidity  at  the  same  time. 

The  measurement  of  absolute  humidity  by  direct  methods  is 
not  ordinarily  required  in  weather  observations.  It  may  be 
determined  by  aspirating  a  measured  quantity  of  air  very  slowly 
through  a  Hygroscopic  substance,  weighing  the  substance  before 
and  after.  It  may  be  determined  more  easily,  however,  by  ascer- 
taining the  relative  humidity. 

The  Measurement  of  Humidity;  Wet-Dry-Bulb  Hygrom- 
eters.— Various  methods  of  determining  the  humidity  of  the 
air  have  been  devised.  Some  of  them  are  merely  hygroscopes. 
Thus,  a  slip  of  paper  moistened  with  a  solution  containing 
gelatine  and  cobaltic  chloride  is  pink  in  moist  air  and  blue  in 
dry  air.  A  better  hygroscope  is  the  toy  chalet  from  which  a 
woman  emerges  in  dry  weather,  while  a  man  with  an  umbrella 
stands  in  the  door  during  damp  weather.  The  manikins  are 
suspended  by  a  short  piece  of  catgut  which,  twisting  in  the  one 
case  and  untwisting  in  the  other,  because  of  changing  moisture 
content,  indicates  roughly  the  changes  in  the  humidity  of  the  air. 

A  curled  piece  of  vegetable  fiber,  one  end  fastened  to  a  base, 
the  other  carrying  an  index  hand,  has  become  popular  as  a 
hygroscope.  It  has  practically  no  value  for  quantitative  deter- 
minations, but  is  not  without  value  in  indicating  conditions  of 
moisture  not  at  once  apparent  to  the  senses  but  at  the  same  time 
necessary  to  bodily  comfort. 

For  the  quantitative  measurement  of  humidity  hygrometers 
are  now  practically  reduced  to  two  types,  dry-wet-bulb  ther- 
mometers, and  hair  hygrometers.  The  United  States  Weather 
Bureau  provides  several  kinds  of  the  first-named  type. 


216       MEASUREMENT   OF    HUMIDITY:     HYGROMETERS 


The  Mason  hygrometer,  nearly  two  hundred  years  old  in 
principle,  has  been  a  standard  in  all  countries  for  many  years. 
It  is  made  in  various  forms,  but  the  essential  features  do  not 

vary.  The  instrument  consists 
of  two  thermometers  mounted  on 
the  same  base.  One  measures  the 
temperature  of  the  free  air;  the 
bulb  of  the  other  is  covered  with 
a  single  thickness  of  thin  bolting 
silk  or  muslin,  the  lower  end  of 
which  is  in  a  small  vessel  of  water 
attached  to  the  base  board.  Capil- 
lary attraction  keeps  the  fabric 
wet  and  evaporation  is  almost 
always  taking  place. 

The  evaporation  of  the  water 
chills  the  bulb  and  the  wet-bulb 
thermometer  therefore  registers 
a  lower  temperature.  The  more 
rapidly  the  evaporation  takes 
place,  the  less  is  the  moisture 
content  of  the  air;  and  the  per- 
centage may  be  determined  by 
the  difference  of  the  readings  of 
the  two  thermometers.  Tabu- 
lated determinations  accompany 
the  hygrometer,  and  the  percent- 
age of  moisture  already  calculated 
is  found  from  tables  contained 
in  the  Weather  Bureau  Circular 
of  Instruction. 

The  hygro-autometer  is  a  very 
convenient  form  of  the  Mason 
hygrometer.  The  tables  are 
carried  on  a  roll  attached  to  the 


Mason  Hygrometer.     Weather 
Bureau  pattern. 


hygrometer.  Thumbscrews  turn  the  rolls  until  the  difference 
between  air  temperature  and  wet-bulb  temperature  appears 
in  the  space  at  the  top;  the  per  cent  of  humidity  is  opposite 
the  air  temperature  reading.  The  hygro-autometer  is  a  most 
excellent  hygrometer  for  auditoriums  and  for  household  use. 


TYPES  OF  HYGROMETER  217 

The  hygrodeik  is  a  form  of  hygrometer  in  which  the  tabulated 
matter  is  shown  on  a  card  ruled  with  ordinates  and  co-ordinates 
for  the  convenience  of  reading.  An  index  fastened  by  a  hinge 
joint  at  the  top  carries  also  a  sliding  point.  By  the  adjustment 
of  these  the  relative  humidity  is  read  from  the  tabulated  figures. 
The  experience  of  nearly  a  century  has  shown  the  usefulness  of 
this  instrument  for  indoor  purposes. 

Unless  "  coaxed  "  by  fanning,  dry- wet-bulb  calculations  are 
subject  to  error,  the  nature  of  which  is  obvious.  The  use  of  an 
ordinary  fan — or,  better,  an  electric  fan — on  the  bulbs  will 
give  much  more  accurate  readings. 

The  sling  psychrometer  obviates  this  difficulty.  The  two 
thermometers  of  this  instrument  are  made  fast  to  a  metal 
strip  which  whirls  upon  a  pivoted  handle.  The  covered  bulb  is 
dipped  in  water  of  the  same  temperature  as  the  air  and  whirled 
on  the  pivot  until  the  temperature  of  the  wet  bulb  ceases  to 
lower.  Ordinarily,  about  twenty  seconds  are  required  to  obtain 
a  correct  reading. 

The  whirling  table  is  now  generally  employed  where  system- 
atic observations  are  made.  The  geared  mechanism  used  in 
whirling  the  thermometers  does  not  give  more  accurate  results 
than  the  sling  psychrometer,  but  it  affords  an  easier  method 
of  stimulating  evaporation,  and  the  thermometers  are  not  so 
likely  to  be  broken. 

When  the  humidity  of  the  air  is  near  the  point  of  saturation, 
determinations  made  at  the  same  time  may  vary  several  points; 
and,  unless  a  sling  psychrometer  or  a  whirling  apparatus  is 
used,  the  determinations  are  pretty  certain  to  vary.  The 
thermometer  scales  of  the  best  psychrometers  are  graduated  to 
half-degrees  and  may  be  read  to  quarter-degrees.  This  conduces 
materially  to  accuracy. 

The  chief  source  of  inaccuracy,  however,  is  the  covering  of 
the  wet-bulb  thermometer.  No  matter  what  the  material  of 
which  it  is  constructed  may  be,  sooner  or  later  it  becomes  hard 
and  loses  its  capillarity.  It  is  no  longer  of  use  and  should  be 
thrown  away.  If  it  shows  signs  of  discoloration  it  should  be 
thrown  away  also,  for  its  usefulness  is  gone.  Tubular  wicks 
are  now  much  used  and  are  kept  by  dealers  in  meteorological 
instruments.  If  any  doubt  as  to  the  cleanliness  of  a  wick  exists, 
it  should  be  boiled  briskly  in  water  and,  when  dry,  soaked  in 


218       MEASUREMENT    OF    HUMIDITY:     HYGROMETERS 

pure  alcohol  in  order  to  remove  any  traces  of  grease.  Under  no 
circumstances  should  gasoline  be  used.  In  adjusting  it  to  the 
bulb,  every  trace  of  oil,  grease,  or  other  substance  should  be 
removed  from  the  fingers.  If  the  capillarity  of  the  wick  is 
sluggish  it  is  better  to  try  another.  In  an  emergency,  if  the 
fabric  about  the  bulb  is  dry  it  may  be  wet  with  a  camel's  hair 
brush  dipped  in  water.  Water  containing  any  sort  of  impurity 
is  apt  to  impair  the  capillarity  of  the  wick  or  even  destroy  it. 
In  "  hard  water  "  localities  distilled  water  would  better  be  used. 
In  catching  rain  water  it  is  well  to  bear  in  mind  that  the  water 
falling  during  the  first  part  of  a  shower  may  be  very  dirty. 

When  the  moisture  is  close  to  saturation  two  or  three  suc- 
cessive determinations  may  be  necessary  for  a  satisfactory 
result.  When  the  humidity  is  very  low  the  difference  between 
the  dry-  and  the  wet-bulb  reading  of  several  determinations  may 
be  considerable.  In  this  case,  too,  the  observer  must  use  his 
judgment.  A  mean  of  several  determinations  is  a  fairly  safe 
record. 

When  the  water  in  the  cup  of  the  Mason  hygrometer  is 
frozen,  care  should  be  used  in  making  the  reading.  If  the  upper 
end  of  the  wick  is  dry — sometimes  this  is  the  case — the  deter- 
mination may  be  regarded  with  suspicion.  It  is  better  to  wet 
the  part  around  the  bulb  by  means  of  a  camel's  hair  brush  and 
wait  a  few  minutes  until  the  thermometer  has  settled  to  a  fixed 
temperature  before  reading.  A  sling  psychrometer  gives  a 
more  accurate  result  in  freezing  weather,  and  its  use  is  more 
convenient.  The  wick,  or  covering,  may  be  wet  with  water  at 
ordinary  temperature,  but  the  whirling  must  be  continued  until 
no  further  reduction  of  the  wet-bulb  temperature  occurs. 

The  Hair  Hygrometer. — Human  hair  freed  from  its  natural 
oil  and  from  grease  of  every  sort,  is  highly  sensitive  to  moisture. 
It  may  be  made  chemically  clean  by  a  bath,  first  in  water  with 
a  mild  soap  and,  after  drying,  in  ether.  After  the  ether  bath 
it  should  not  come  in  contact  with  bare  hands.  If  one  end  of 
a  clean  hair — or  a  strand  of  several  hairs — be  made  fast  to  a 
binding  post  and  the  other  wound  around  the  axle  of  a  dial 
needle  and  kept  taut  by  a  spring,  the  lengthening  and  shortening 
of  the  hair  by  changing  moisture  may  be  made  to  indicate 
humidity  with  a  fair  degree  of  accuracy. 

Commercial  hair  hygroscopes  and  hair  hygrometers  of  various 


HAIR  HYGROMETERS  219 

forms  are  now  made  by  several  manufacturers.  Those  of  the 
best  quality  retain  the  name  of  the  manufacturer;  others  are 
stamped  with  the  name  of  the  retailer.  When  new  and  clean, 
those  of  the  best  quality  are  about  equal  in  accuracy  to  the 
psychrometer.  Hair  hygrometers  usually  deteriorate  with 
continued  use.  The  chief  trouble  comes  from  dust,  and  from 
gumming  or  fouling  of  bearings.  Like  any  other  delicate  mech- 
anism, careful  cleaning  and  oiling  will  prolong  the  life  of  a  hair 
hygrometer  and  preserve  the  accuracy  of  its  registration.  In 
the  case  of  a  commercial  hygrometer,  when  once  it  has  gone 
wrong,  it  is  usually  less  expensive  to  purchase  a  new  instru- 
ment than  to  repair  an  old  one. 

In  spite  of  its  shortcomings,  the  convenience  of  the  hair 
hygrometer  outweighs  its  disadvantages.  For  use  in  dwellings, 
school-rooms,  textile  establishments,  candy  factories  and 
tobacco  factories  it  is  far  better  than  the  ordinary  Mason  type 
of  instrument.  The  humidity  is  read  instantly;  computation 
from  reference  tables  is  not  required. 

The  hygrograph  is  a  hygrometer  with  recording  mechanism 
like  that  of  the  thermograph.  Drum,  clock  and  record  sheets 
are  much  the  same  in  both,  except  that  the  record  sheets  of 
the  hygrograph  show  per  cent  values  in  their  horizontal  rulings. 
The  instrument  is  delicate  and  very  sensitive  to  changes.  Its 
records  are  not  always  trustworthy,  but  its  errors  are  readily 
checked  and  adjustments  are  easily  made.  It  should  be  sheltered 
so  that  by  no  possibility  can  rain  or  snow  be  driven  upon  it. 

A  hygrograph  is  usually  a  part  of  the  equipment  of  each 
Weather  Bureau  station.  In  spite  of  the  difficulties  of  trans- 
portation it  is  a  very  useful  instrument  in  field  stations.  It  is 
useful  not  only  in  noting  the  changes  in  relative  humidity;  it 
also  may  be  an  indication  of  change  in  absolute  humidity. 

The  normal  movement  of  the  thermograph  pen  is  upward 
from  sunrise  until  3  o'clock,  then  downward  to  the  minimum  of 
the  next  morning;  the  normal  curve  of  the  hygrograph  is 
opposite  in  movement — downward  from  sunrise  to  3  o'clock 
and  then  upward.  These  movements  usually  are  so  regular 
that  experience  enables  one  to  read  temperature  approximately 
from  the  hygrogram  sheet,  and  humidity  from  the  thermogram. 
But  when  the  temperature  line  is  normal  and  the  humidity 
line  is  abnormal,  a  change  in  the  absolute  humidity  has  occurred. 


220       MEASUREMENT   OF   HUMIDITY:     HYGROMETERS 

Experience  will  teach  the  observer  to  look  for  the  unusual  in 
comparing  the  daily  records,  and  also  to  interpret  it. 

The  dew-point  may  be  found  without  the  use  of  the  hygrom- 
eter by  a  very  practical  method.  A  thermometer,  a  polished 
tin  cup — a  "  shaker  "  is  better — and  ice  water  are  required. 
The  cup  must  be  absolutely  free  from  grease.  The  cup  is  half- 
filled  with  water  at  about  the  temperature  of  the  air.  Ice  water 
is  added  little  by  little  and  stirred  with  the  thermometer  until 
mist  forms  on  the  outside  pf  the  cup.  The  water  is  then  at  the 
temperature  of  the  dew-point,  and  this  is  shown  by  the  ther- 
mometer. 

The  Measurement  of  Evaporation;  Evaporimeters. — The 
rate  of  evaporation  depends  on  the  amount  of  moisture  in  the 
air.  If  the  humidity  is  low,  evaporation  is  more  rapid  than 
when  it  is  high.  With  the  humidity  above  95  per  cent  a  piece 
of  wet  muslin  in  the  open  air  may  require  more  than  an  hour 
to  dry;  with  the  humidity  at  100  per  cent  it  does  not  dry  at 
all.  A  high  temperature  also  favors  evaporation — chiefly  from 
the  fact  that,  with  rising  temperature,  the  relative  humidity 
decreases  without  any  change  in  absolute  humidity.  The  rate 
of  evaporation  is  therefore  roughly  an  indication  of  the  degree 
of  humidity. 

Evaporimeters  vary  in  form  but  not  in  principle.  In  every 
case  the  evaporimeter  is  a  device  for  measuring  the  depth  of 
water  lost  by  evaporation  from  an  open  surface.  A  very  common 
form  consists  of  a  graduated  glass  tube  filled  with  water,  inverted 
in  a  vessel  of  water.  A  pin-hole  aperture  about  half  an  inch 
from  the  lower  end  of  the  tube  admits  air  to  the  top  of  the  tube 
when  evaporation  lowers  the  water  in  the  level  of  the  pan.  The 
level  of  the  water  in  the  pan  is  constant ;  the  loss  is  in  the  tube. 
If  the  area  of  the  surface  of  the  container  is  o.oi  that  of  the 
section  of  the  tube,  a  loss  of  I  inch  of  water  in  the  tube  is  equiva- 
lent to  a. loss  of  o.oi  inch  by  evaporation. 

The  Piche  evaporimeter  is  a  type  of  the  best  sort  of  instru- 
ment. A  collar  fastened  around  the  tube  at  its  mouth  carries 
a  disk  which  presses  against  and  covers  the  mouth  of  the  tube. 
A  circular  piece  of  filter  paper,  about  twice  the  diameter  of  the 
tube,  between  the  disk  and  the  mouth  of  the  tube  allows  a 
sufficient  flow  of  water  to  keep  the  paper  wet.  By  this  device, 
loss  of  water  by  accident  is  avoided,  and  evaporation  is  recorded 


EVAPORIMETERS  221 

with  a  fair  degree  of  accuracy.  The  Weather  Bureau  issues 
evaporimeters  when  they  are  deemed  a  necessary  part  of  the 
equipment  of  a  station.  Field  evaporimeters  of  different  pat- 
terns, each  for  a  specific  use,  are  used  at  various  Weather  Bureau 
stations. 

A  bottle  with  straight  sides  may  be  inverted  in  a  pan  and 
used  as  an  evaporimeter  in  an  emergency.  If  the  area  of  the 
section  of  the  bottle  be  determined,  an  approximate  rate  of 
evaporation  may  be  found.  It  is  better  to  have  the  mouth  of 
the  bottle  rest  on  circular  pieces  of  blotting  paper,  several 
thicknesses  being  used.  The  flow  of  water  through  the  blotting 
paper  will  be  kept  fairly  steady  by  the  admission  of  bubbles 
of  air.  It  is  hardly  necessary  to  add  that  values  obtained  thus 
are  only  approximates. 


CHAPTER  XIX 

THE   MEASUREMENT   OF   PRECIPITATION:   RAIN 
GAUGES:   SNOW  MEASUREMENT 

All  moisture  condensed  from  the  air — rain,  snow  or  hail — 
is  classed  as  "  precipitation  "  and  is  measured  in  terms  of  rain. 
It  represents  the  depth  of  water  which  would  accumulate  on  a 
level  surface  without  loss  by  run-off,  percolation  or  evapora- 
tion. For  convenience  in  measurement  the  water  of  precipita- 
tion— rain,  or  melted  snow — is  caught  in  vessels  of  special  con- 
struction called  rain  gauges.  These  are  of  various  forms,  but 
they  have  practically  the  same  principle — the  exhibition  of  a 
depth  of  rain  expressed  in  inches  and  hundredths. 

The  standard  Weather  Bureau  rain  gauge  is  a  cylindrical 
barrel  of  galvanized  iron  26  inches  in  height  over  all.  The 
receiver  is  a  funnel  with  an  upright  collar  of  bronze  2  inches 
high.  The  edge  of  the  collar  is  beveled  so  as  to  present  a  sharp 
cut-water  to  the  rain.  The  receiver  delivers  the  water  to  the 
measuring  tube ;  a  sleeve  at  the  lower  end  of  the  funnel  holds  the 
mouth  of  the  measuring  tube  in  place  and  prevents  the  loss  of 
water.  The  brass  measuring  tube,  accurately  calibrated,  rests 
in  a  seat  at  the  bottom  of  the  barrel. 

The  receiver  is  exactly  8  inches  in  diameter;  the  section  of 
the  measuring  tube  is  exactly  one-tenth  the  area  of  the  receiver. 
Its  diameter  is  2.53  inches  and  it  is  20  inches  high.  An  inch  of 
rain  therefore  measures  10  inches  in  the  tube,  and  the  latter 
holds  2  inches  of  rain.  Any  excess  beyond  2  inches  overflows 
into  the  larger  vessel;  it  is  poured  into  the  emptied  measuring 
tube  and  added  to  the  amount.  The  measuring  stick  is  gradu- 
ated to  measure  inches,  tenths,  and  hundredths,  and  the  depth 
of  the  rainfall  is  indicated  by  the  length  wetted  when  the  stick 
is  inserted  in  the  measuring  tube.  Care  must  be  used  to  keep 
the  surface  of  the  measuring  stick  free  from  grease.  If  it  fails 

222 


RECORDING  AND  REGISTERING  GAUGES 


223 


to  show  the  wet-line  clearly  the  surface  may  be  cleaned  with 
alcohol,  or — better — rubbed  clean  with  oo  sandpaper. 

Recording  and  Registering  Gauges. — The  registering  and 
recording  gauges  are  mainly  of  two  classes — float-gauges,  in 
which  the  increasing  depth  of  water,  by  lifting  a  float  balanced 
by  a  weight  translates  motion  to  a  pen  arm;  and  the  tipping- 
bucket  gauges,  in  which  each  tip  of  a  full  bucket  moves  an  index 
hand. 


a 

V        A 

f- 

d 

1 

d 

Wi 
J  e 

B 

•c 

B 

VERTICAL   SECTION          HORIZONTAL  SECTION,  E.  F. 


Standard  Weather  Bureau  rain  gauge:  A,  receiver;  B,  barrel;   C,  measuring 

tube. 

The  Marvin  float  gauge,  used  at  many  stations,  is  provided 
with  a  wind  shield  of  the  Nipher  type,  about  21  inches  square. 
The  drum  carries  a  sheet  ruled  with  lines  nearly  horizontal, 
but  inclined  so  that  they  form  a  continuous  spiral.  These  lines 
carry  the  record,  one  for  each  day  of  the  week.  Vertical  lines 
divide  the  sheet  into  ten-minute  spaces.  The  drum,  driven  by 
clockwork,  makes  one  revolution  in  twenty-four  hours.  A 
screw  thread  of  the  required  pitch  causes  the  recording  pen  to 
follow  the  spaces  between  the  spiral  lines  on  the  record  sheet. 

When  rain  begins  to  gather  in  the  measuring-tube,  the  lifting 
of  the  float  causes  the  rotation  of  the  cam  shaft  and  this  imparts 
a  lateral  motion  to  the  pen.  The  graph  made  by  the  pen  con- 


224     MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES 

sists  of  a  sinuous  line  which  curves  back  and  forth  across  the 
day  line.  A  complete  revolution  of  the  cam  records  half  an  inch 
of  rain.  The  faster  the  precipitation,  the  sharper  are  the  curves. 

The  Marvin  gauge  possesses  several  distinct  advantages. 
Recording  begins  within  a  very  few  minutes  after  precipitation 
has  commenced — a  merit  not  possessed  by  tipping-bucket 
gauges;  it  likewise  records  the  cessation  of  rainfall  rather  more 
promptly.  A  very  desirable  feature  is  the  fact  that  it  also 
records  the  rate  of  rainfall,  a  matter  of  great  importance.  In 
many  instances  the  total  of  precipitation  is  of  minor  conse- 
quence, while  the  rate  per  unit  of  time  must  determine  the 
discharging  capacity  of  sewers  and  other  run-off  systems.  The 
Marvin  gauge  is  not  fool-proof,  but  this  detail  applies  also  to 
other  recording  instruments. 

The  tipping-bucket  gauge  is  chiefly  used  for  recording  rain- 
fall. The  drip  from  the  funnel  falls  into  one  or  the  other  of 
two  scoop-shaped  buckets  placed  back  to  back  mounted  on 
trunnion  bearings.  When  o.oi  inch  of  rain  has  collected  in  a 
bucket  the  weight  causes  it  to  tip,  spilling  the  water  into  a 
container  and  moving  a  pointer  one  division  on  a  dial.  The 
tipping  of  the  full  bucket  swings  the  empty  bucket  into  a  posi- 
tion where  it  catches  the  drip. 

Where  a  Friez  triple  recorder  is  used  the  pen  which  ordinarily 
records  sunshine  is  aiso  used  to  record  rainfall.  This  it  does  with 
little  or  no  confusion  of  records,  because  precipitation  rarely 
occurs  in  appreciable  amount  while  the  sun  is  shining. 

Tipping-bucket  rain  gauges  are  constructed  so  close  to 
exactness  of  measurement  that,  when  placed  side  by  side  with 
the  standard  gauges,  the  difference  between  the  measurements 
of  the  two  is  not  much  greater  than  that  of  two  gauges  of  the 
same  type  side  by  side. 

Although  the  tipping-bucket  rain  gauges  are  simple  in  con- 
struction, various  conditions  may  occur  that  result  in  erroneous 
recording.  The  bucket  may  rebound  on  emptying  itself,  in 
which  case  two  registrations  instead  of  one  are  made.  With  the 
Friez  gauge  this  will  appear  as  a  mark  of  double  length  on  the 
record.  With  the  dial  gauges,  which  register  but  do  not  record 
graphically,  double  registration  cannot  be  discovered  except 
by  close  watching.  It  may  be  suspected,  when  the  catchment  of 
the  registering  gauge  runs  uniformly  greater  than  that  of  the 


TIPPING-BUCKET  RAIN  GAUGES  225 

standard  Weather  Bureau  gauge.  In  the  case  of  the  Friez 
gauge  a  readjustment  of  the  stop  pins  is  necessary.  With  the 
Short  and  Mason  gauge  a  pressure  brake  is  provided;  the  tight- 
ening of  this  will  prevent  back-tipping. 

Sometimes  it  happens  that  the  stick  measurement  of  the 
catch  is  considerably  in  excess  of  that  registered  on  the  dial  or 
•  recorded  on  the  paper;  indeed,  this  is  pretty  apt  to  be  the 
case  in  heavy  summer  showers.  Usually  this  discrepancy  is 
due  to  the  fact  that  a  measurable  interval  of  time  elapses  while 
the  bucket  is  discharging  its  load  of  water,  after  it  has  been 
filled.  The  water  that  flows  into  the  bucket  during  the  interval 
is  therefore  slightly  in  excess  of  the  normal  o.oi  inch.  During 
a  very  heavy  shower,  aggregating  3  inches,  the  excess  of  the 
measured  amount  over  the  recorded  amount  may  be  as  great 
as  0.15  inch.  In  such  a  case  stick  measurement  rather  than 
bucket  measurement  should  be  taken  as  the  total  precipitation. 

The  drip  aperture  of  the  Short  and  Mason  registering  gauge 
is  very  small;  although  protected  against  clogging  by  falling 
leaves,  it  may  become  clogged  with  dust.  Under  such  circum- 
stances the  funnel  may  fill  and  overflow  with  no  water  running 
into  the  buckets.  If  the  gauge  receives  even  ordinary  care  such 
a  condition  is  not  likely  to  occur.  Cleaning  the  receiver  daily  is 
not  absolutely  essential,  but  a  conscientious  observer  will  see 
that  the  gauge  is  always  in  order. 

The  inside  of  a  gauge  is  a  spot  most  dear  to  the  heart  of  the 
spider,  and  in  many  a  case  the  accumulation  of  spider  web 
has  tied  up  the  registering  mechanism  so  completely  that 
registration  ceased. 

Another  source  of  annoyance  in  registering  and  recording 
tipping-bucket  gauges  has  to  do  with  the  condition  of  the 
buckets.  In  regions  where  the  air  is  very  dirty,  sediment  may 
cling  to  the  surface  of  the  bucket,  and,  adding  to  its  weight 
unequally  on  opposite  sides,  prevent  a  true  registration.  A 
still  greater  source  of  error  may  result  from  handling  the  inner 
surface  of  the  buckets  with  greasy  fingers.  The  water  will  not 
cohere  to,  or  "wet"  a  greasy  surface;  and  this  may  cause  a 
slight  but  persistent  error  in  registration. 

It  is  not  wise  to  depend  wholly  upon  a  registering  or  a  record- 
ing gauge;  stick  measurement  is  more  certain.  Nevertheless,  a 
station  of  any  sort  should  be  provided  with  two  gauges,  and  a 


226     MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES 


registering  gauge  is  a  most  excellent  feature  of  equipment. 
H.  J.  Green  makes  an  indoor  dial  that  may  be  attached  readily 
to  any  tilting-bucket  gauge.  Such  a  device  is  very  convenient. 

Intensity  of  Precipitation. — The  intensity  of  rainfall  may  be 
of  greater  importance  than  the  gross  amount.  The  Marvin  and 
the  Friez  gauges  record  intensity  graphically.  The  observer 
with  the  ordinary  gauge  can  find  the  intensity  in  one  way  only 
— by  making  measurements  at  regular  intervals.  During 
ordinary  rainstorms  measurements  made  at  half-hour  intervals 
will  suffice,  and  these  need  be  continued  for  not  more  than 
two  hours.  During  heavy  summer  downpours,  however,  the 
measurements  should  be  made  at  five-minute  intervals. 

To  the  farmer,  2  inches  of  rain  distributed  over  the  greater 
part  of  the  day  means  a  thorough  soaking  of  the  ground;  but 
if  concentrated  within  half  an  hour  it  means  beaten-down 
grain  and  washed-out  ditches.  Such  a  rainfall,  to  the  engineer, 
means  washouts  all  along  the  track;  to  the  city  engineer  it 
means  flooded  sewers  and  excavations.  The  engineer  who  takes 
care  of  drainage  must  know  how  to  guard  against  phenomenal 
rainfalls  by  building  so  as  to  take  care  of  them. 

Observers  will  make  their  work  more  helpful  by  noting  not 
only  the  fact  of  excessive  rainfall  but  also  its  rate  at  five-minute 
intervals.  In  Weather  Bureau  practise,  the  term  excessive 
has  a  technical  application,  and  the  tabulation  of  excessive 
amounts  during  such  intervals  is  required.  The  following  table 
shows  the  intensity  of  precipitation  that  technically  is  excessive. 
It  is  based  on  the  experience  of  many  years. 


1  Duration  in 
minutes 

Depth  in 
inches 

Duration  in 
minutes 

Depth  in 
inches 

5 

10 

15 

0.25 
0.30 
0-35 

35 
40 

45 

0-55 
0.60 
0.65 

20 
25 

0.40 
0-45 

50 
60 

0.70 
0.80 

30 

0.50 

1  At  Porto  Bello,  Panama,  2.48  inches  were  reported  during  a  period  of 
five  minutes,  Nov.  29,  1911,  at  2:07  A.M.  At  Curtea  de  Arges,  Rumania, 
8.07  inches  fell  in  twenty  minutes,  July  7,  1889.  These  are  the  heaviest  rain- 
falls of  record,  but  they  may  have  been  exceeded  by  cloudbursts  in  which 
measurements  were  not  made. 


INTENSITY  OF  PRECIPITATION  227 

When  excessive  rainfall  extends  beyond  a  duration  of  two 
hours  the  measurements  are  recorded  at  fifty-minute  inter- 
vals. 

The  amount  of  rainfall  necessary  to  insure  a  specific  crop 
varies  with  locality  and  with  the  character  of  the  crop.  More 
especially  it  depends  on  the  distribution  of  the  rainfall  over  the 
growing  season.  Roughly,  rain  must  fall  during  a  period  which 
covers  three-quarters  or  more  of  the  growing  season  for  the 
particular  crop.  The  growing  season  for  wheat  is  over,  in  most 
localities,  by  the  middle  of  July — in  some  localities  by  the  middle 
of  June.  The  growing  season  for  corn  extends  into  September. 
A  rainfall  of  12  inches,  fortunately  distributed,  may  be  all  that 
is  required  for  a  specific  crop.  Unfortunately  distributed,  a 
fall  two  or  three  times  as  great  may  not  suffice. 

From  the  nature  of  the  case,  the  knowledge  which  concerns 
crop  safety  must  be  gathered  locally.  Through  its  Climato- 
logical  Service,  the  Weather  Bureau  is  gathering  knowledge  of 
this  sort,  but  additional  information  is  very  desirable.  The 
observer,  whether  official  or  volunteer,  can  aid  in  gathering 
useful  information  along  the  following  lines: 

The  length  of  the  growing  season — that  is,  the  number  of 
days  between  late  spring  frosts  and  early  fall  frosts. 

The  months  during  which  rain  is  necessary  for  each  specific 
crop. 

The  duration  of  droughts — that  is,  the  number  of  days  during 
which  no  rain  or  only  a  trace  of  rain  falls — that  are  hurtful  or 
destructive  to  specific  crops. 

The  character  of  soil  with  respect  to  rainfall  necessary  to 
crop  growth. 

As  a  rule  the  precipitation  records  of  the  nearest  Weather 
Bureau  station — regular  or  cooperative — will  furnish  the  neces- 
sary information  concerning  the  amount  of  precipitation.  The 
specific  locality  sometimes  requires  its  own  rain  measurements. 
A  rain  gauge  of  the  Weather  Bureau  pattern  is  useful,  but  a 
metal  container  with  straight  sides  will  answer  fairly  well,  and 
an  inch  rule  will  answer  the  purpose  of  a  measuring  stick.  The 
volunteer  observer  who  studies  the  rainfall  of  a  locality  may 
thus  gain  the  essential  information  required;  namely,  the 
minimum  amount  of  rain,  and  also  the  optimum  rainfall  both 
as  to  amount  and  distribution,  for  a  specific  crop. 


228     MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES 


LOCATION  OF  THE  RAIN  GAUGE  229 

The  Location  of  the  Rain  Gauge. — In  establishing  any  sort 
of  a  station  where  the  measurement  of  rainfall  is  to  be  recorded, 
at  least  two  rain  gauges  are  desirable.  One  of  these  should  be 
a  standard  Weather  Bureau  gauge  or  one  of  similar  pattern; 
the  second  may  be  any  vessel  with  an  8-inch  circular  opening 
in  the  cover. 

In  cities  which  are  solidly  built  the  flat  roof  of  a  building 
offers  about  the  only  suitable  place  for  a  rain  gauge.  If  the 
edge  of  the  roof  is  a  parapet,  so  much  the  better,  for  the  drive 
of  the  wind  is  less  apt  to  blow  aside  the  rain  that  should  fall 
into  the  receiver.  In  a  position  of  this  sort  the  catchment  of 
the  two  gauges  is  not  likely  to  differ  materially. 

In  suburban  localities  and  in  communities  where  buildings 
are  100  feet  apart  the  gauges  are  better  placed  in  such  positions 
as  have  the  full  sweep  of  the  rain-bearing  winds.  If  two  places 
100  feet  or  more  apart  show  no  material  difference  in  the  catch, 
either  location  is  probably  suitable.  With  gentle  rain  and  still 
air  the  two  gauges  should  be  in  close  agreement;  if  the  wind 
blows  in  strong  gusts  there  may  be  a  material  difference. 

The  wind  is  the  chief  obstacle  to  accuracy  of  rainfall  measure- 
ment and  shielding  the  gauge  from  the  full  strength  of  the 
wind  is  the  best  means  to  insure  an  accurate  catch.  The  Nipher 
shield  is  a  trumpet-shaped  metal  device  about  20  inches  across 
which  surrounds  the  mouth  of  the  gauge.  It  is  surmounted  by 
a  rim  of  copper  mesh  which  prevents  insplashing.  J.  O.  Alter, 
Observer  at  Salt  Lake  City,  constructed  a  much  simpler  shield 
by  fastening  a  strip  of  canvas  about  9  inches  wide,  to  a  metal 
ring  about  30  inches  diameter.  The  screen  thus  constructed  is 
suspended  about  the  gauge  by  metal  struts.  The  edge  is  about 
2  inches  higher  than  the  mouth  of  the  gauge.  The  Weather 
Bureau  regards  this  shield  with  favor.  The  author  has  found 
a  similar  shield  made  of  copper  mesh,  such  as  is  used  in  window 
screens,  a  most  excellent  device.  In  the  long  run,  a  shielded 
gauge  will  catch  from  6  per  cent  to  10  per  cent  more  rain  than 
one  unshielded,  according  to  the  experience  of  the  Weather 
Bureau.  P.  R.  Jameson,  with  measurements  covering  many 
years,  finds  a  gain  of  about  9  per  cent  in  the  case  of  shielded 
gauges. 

The  pit-gauge  is  favorably  considered  by  C.  F.  Marvin, 
an  authority  on  precipitation.  The  pit-gauge  is  merely  a 


230     MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES 

depression  in  which  a  standard  Weather  Bureau  gauge  is  placed 
so  that  its  mouth  is  10  or  12  inches  higher  than  ground  surface. 
It  is  surrounded  by  a  rim  of  earth  in  the  form  of  a  ring  about 
6  feet  in  diameter.  A  pit  and  ring  of  concrete  with  a  movable 
cover  of  wire  mesh,  coarse  enough  to  permit  rain  to  enter  without 
obstruction  and  fine  enough  to  keep  leaves  out  makes  an  ideal 
position  for  a  rain  gauge  in  an  open  and  fairly  level  country. 

The  Ferguson  gauge  designed  for  isolated  stations  by  S.  P. 
Ferguson,  of  the  United  States  Weather  Bureau,  totalizes  a 
year's  rainfall  month  by  month,  or  in  such  measured  propor- 
tions as  may  be  desired.  A  film  of  oil  in  each  of  the  thirteen 
receivers  prevents  loss  by  evaporation. 

The  Measurement  of  Snow. — A  reasonably  accurate  meas- 
urement of  snowfall  is  desirable  in  regions  of  plentiful  rainfall; 
it  is  imperative  in  regions  where  the  irrigation  of  crops  or  a 
knowledge  of  possible  floods,  or  of  droughts  is  a  prerequisite. 

In  moderately  level  regions  of  gentle  drainage  the  measure- 
ment of  the  precipitation  derived  from  snow  requires  the 
care  and  judgment  that  comes  only  with  experience.  In  moun- 
tainous regions  it  requires  judgment,  patience  and  a  lot  of  hard 
work. 

On  the  prairies  of  Indiana,  for  instance,  the  measurements 
made  at  the  Weather  Bureau  stations  give  a  pretty  accurate 
total  of  precipitation.  If  the  aggregate  error  amounted  to  12 
inches  of  snow,  or -even  2  inches  of  rain,  however,  the  result 
would  not  be  materially  harmful.  In  California,  however,  the 
floods  of  the  Sacramento  and  San  Joaquin  River  valleys  are 
largely  predetermined  by  the  snowfall  in  the  mountain  slopes 
to  the  eastward;  and  in  many  arid  regions  the  crop  production 
which  depends  on  irrigation  must  be  foretold  mainly  by  the 
total  of  snowfall. 

In  level  regions  where  the  snow  is  not  disturbed  by  the  wind 
the  measurement  is  not  difficult.  The  observer  uses  his  meas- 
uring stick  in  a  dozen  or  more  places  within  a  radius  of  300 
feet.  Usually  the  mean  depth  will  become  apparent  without 
computation. 

To  find  the  equivalent  in  terms  of  rainfall  requires  "  putter- 
ing and  patience."  A  very  convenient  way  is  to  cut  a  section 
in  the  snow  with  the  inverted  barrel  of  the  standard  rain  gauge, 
thrusting  a  dust  pan  or  a  piece  of  sheet  iron  under  the  mouth 


MEASUREMENT  OF  SNOW 


231 


in  order  to  hold  the  section  firmly.  It  is  advisable  to  cut  at 
least  three  sections.  The  melted  snow  may  then  be  measured 
in  the  tube,  taking  one-third  of  the  total.  Melting  the  snow 
may  be  expedited  by  pouring  into  the  barrel  containing  the 
snow  a  measuring  tube 
exactly  full  of  hot 
water,  thereby  reduc- 
ing the  snow  to  a 
condition  sufficiently 
liquid  to  be  measured. 
Two  inches  must  be 
deducted  for  the  water 
added;  one-third  of 
the  remainder  is  the 
depth  of  equivalent 
rainfall. 

In  mountain  re- 
gions where  the  depth 
of  a  single  fall  may 
be  several  feet,  such  a 
method  of  reduction 
is  out  of  the  question. 
Several  convenient  ex- 
pedientsareemployed. 
A  gauge  40  inches 
high  with  an  interior 
diameter  of  10  inches 
provided  with  a 
Nipher  shield,  is  used 
at  the  station  where 
not  less  than  two  ob- 
servations a  day  are 
made.  The  accumu- 
lation of  snow  is 
weighed  from  time  to 
time  on  a  spring  bal-  The  Marvin  shielded  seasonal  snow-gauge. 

ance,  the  dial  of  which 

reads  hundredths  of  an  inch  instead  of  ounces.  A  mechanical 
device  lifts  the  receiver  from  its  support  so  that  it  can  be  readily 
removed  to  the  swinging  arm  that  carries  the  balance.  This 


232     MEASUREMENT  OF  PRECIPITATION:    RAIN  GAUGES 

is  about  the  most  expeditious  method  of  measuring  snowfall 
yet  devised. 

In  the  western  slope  of  the  Sierra  Nevada  Mountains  "  sea- 
sonal gauges,"  with  collectors  large  enough  to  hold  the  ac- 
cumulation of  snow  and  rain  for  several  weeks,  are  employed. 
The  catch  is  weighed  at  convenient  times. 

When  deep  snowfalls  occur  the  Marvin  snow  tube  has  been 
found  a  most  convenient  device.  This  tube,  as  improved  by 
Church,  is  made  of  galvanized  iron  and  is  2.75  inches  in  diameter. 
The  upper  end  is  left  open;  the  lower  end  is  reinforced  by  a 
piece  of  tubing  forced  inside  the  measuring  tube.  The  lower 
edge  of  the  tube  is  serrated  with  teeth  like  those  of  a  cross- 
cut saw  in  order  to  facilitate  boring  through  crusted  snow  and 
sheets  of  ice. 

Tube  and  core  are  weighed  by  a  spring  balance  that  records 
inches  and  hundredths.  The  tube  has  also  an  engraved  scale 
to  show  the  depth  of  snow.  Church,  working  in  the  Sierra 
Nevada  ranges,  used  the  tube  in  snow  accumulations  30  feet 
thick. 

The  Marvin  density  bucket  provides  a  quick  and  accurate 
method  of  obtaining  the  rain  equivalent.  The  copper  bucket 
is  inverted  and  pressed  lightly  into  the  snow  until  the  top  of 
the  snow  touches  the  bottom  of  the  bucket.  The  bucket,  even 
full  of  snow,  is  then  weighed  on  the  accompanying  scales,  which 
are  graduated  to  per  cent  of  weight  of  an  equal  volume  of  water. 
Thus,  if  the  net  weight — the  total  weight  minus  that  of  the 
bucket — is  0.12  on  the  scales,  it  means  that  the  snow  is  12  per 
cent  of  an  equal  volume  of  water.  Assuming  that  the  depth 
of  snow  is  20  inches,  0.12X20,  or  2.40  inches  is  the  equivalent 
depth  of  rainfall. 

In  mountain  regions  the  intensity  of  precipitation  varies 
with  altitude.  On  the  western  slopes  of  the  high  cordillera  of 
the  Pacific  coast,  McAdie  found  the  greatest  intensity  of  pre- 
cipitation between  4000  and  5000  feet  above  the  valley  floor. 
On  the  eastern  slope  the  intensity  decreased  irregularly  with 
decreasing  altitude.  It  may  be  incautious  to  assume  this  to  be 
true  elsewhere,  but;  as  a  general  truth,  the  basis  of  assumption 
is  not  unreasonable.  It  is  safe  to  assume  that  the  measurement 
of  precipitation  of  the  montane  part  of  a  watershed  must  extend 
from  its  upper  limit  to  the  valley  floor. 


MEASUREMENT  OF  SNOW  233 

A  multiplicity  of  snow  gauges  is  not  required,  but  with  the 
combined  results  of  gauges,  snow  tubes  and  fixed  measuring 
posts,  a  fair  approximate  of  the  catchment  of  the  basin  may  be 
obtained. 

In  practically  all  localities  where  snowfall  requires  measure- 
ment the  following  difficulties  confront  the  observer:  mixed  or 
alternating  snow  and  rain;  rapid  melting  of  snow  while  it  is 
falling;  a  very  light  fall,  say,  less  than  half  an  inch;  rapidly 
drifting  snow.  In  the  case  of  the  first  three,  the  observer  may 
remove  the  receiver  and  tube  from  the  gauge  and  catch  the 
precipitation  in  the  barrel.  In  the  last  case  about  the  only 
way  to  overcome  the  difficulty  is  to  make  a  considerable  number 
of  measurements  where  no  drifting  is  apparent.  One  must  use 
care  to  avoid  measuring  old  snow  with  a  fresh  fall. 


CHAPTER  XX 

THE  MEASUREMENTS  OF  WIND  VELOCITY: 
ANEMOMETERS 

Wind  direction,  wind  velocity  and  the  duration  of  sunshine 
are  usually  recorded  on  the  same  sheet,  each  by  a  separate 
magneto  apparatus.  The  revolving  drum  is  driven  by  a  power- 
ful clock. 

A  record  of  wind  direction  is  required  at  all  observation 
stations  controlled  by  the  various  departments  of  the  govern- 
ment. At  seaports  wind  observations  are  made  the  subject  of 
public  bulletins  and  shipping  interests  are  furnished  not  only 
with  information  to  date  but  also  with  forecasts  of  expected 
changes.  Warnings  of  dangerous  winds  are  sent  broadcast  by 
wireless  for  the  benefit  of  all  vessels  that  may  encounter  them. 

The  requirements  of  air  navigation  are  even  more  exacting 
than  those  of  marine  navigation;  for,  while  the  marine  pilot 
needs  to  know  the  conditions  of  wind  and  weather  at  sea  level, 
the  airman  must  know  them  from  sea  level  to  an  altitude  of 
10,000  feet  or  more. 

Until  recently  Weather  Bureau  stations  have  been  equipped 
with  instruments  for  the  study  of  surface  winds  only.  The 
research  laboratories,  however,  have  made  great  advances  in 
the  study  of  air  conditions  at  considerable  heights,  using  kites 
and  pilot  balloons  carrying  recording  instruments.  Ordinarily 
the  observer  must  depend  on  wind  vanes,  smoke  columns, 
flags,  dust  movements  and  clouds  for  the  determination  of 
wind  conditions. 

Wind  Direction. — The  prevailing  directions  of  the  planetary 
winds  are  discussed  in  another  chapter.  Observers  are  con- 
cerned chiefly  with  the  direction  of  the  wind  at  the  surface. 
Ordinarily  this  does  not  vary  materially  from  the  direction  of 
lower  cloud  movement;  sometimes  it  does  vary,  however, 
and  when  it  does  the  fact  should  be  recorded  as  cross-winds, 

234 


THE  WIND  VANE 


235 


the  direction  of  each  being  noted.  If  the  barometer  is  steady 
and  the  sky  is  free  from  clouds,  the  direction  shown  by  a  wind 
vane  may  be  taken  as  the  direction  of  the  wind  to  the  height 
of  ordinary  flight  altitudes.  Neither  the  surface  winds  nor  the 
cloud  winds  indicate  definitely  the  presence  of  the  updraughts 
and  downdraughts  which  constitute  bumps  and  air  holes. 


Robinson  anemometer,  electrically  connected  with  recording  apparatus, 

P-  239- 

The  Wind  Vane. — The  wind  vane  of  the  spread- tail  pattern, 
used  by  the  Weather  Bureau,  is  probably  the  most  practical 
form  in  use.  It  holds  steadily  to  the  wind;  it  is  sensitive  enough 
to  respond  to  a  breeze  of  2  miles  an  hour.  The  regulation  vane 
is  6  feet  over  all.  The  tail  is  made  of  very  thin  board  strips, 
thoroughly  weather-proofed;  the  metal  work  is  rust-proof; 
the  bearings  are  constructed  so  that  friction  is  reduced  to  the 
minimum.  This  is  the  general  service  vane  designed  to  show 


236  THE   MEASUREMENTS  OF   WIND   VELOCITY 

wind  direction  only.  For  obvious  reasons  it  should  be  mounted 
as  high  above  ground  as  possible,  and  it  should  not  be  in  the 
lee  of  anything  that  may  affect  the  wind  flow. 

The  vane  used  in  connection  with  recording  apparatus  is 
4  feet  in  length.  The  axis  carries  four  partly  overlapping  cam- 
collars  arranged  so  that  at  least  one  collar  is  in  contact  with  the 
electrical  recording  apparatus.  The  latter  prints  a  dotted  line 
on  the  record  sheet.  If  two  cam-collars  are  in  contact  at  the 
same  time  the  intermediate  direction  of  the  wind  is  denoted. 
Thus,  with  both  north  and  west  cams  in  contact,  and  the  pen 
of  each  recording,  the  direction  of  the  wind  is  northwest. 

The  adjustment  of  the  box  containing  the  contact  apparatus 
should  be  made  to  the  geographic  meridian,  and  to  solar  and 
not  standard  time.  Thus,  if  local  time  is  20  minutes  faster 
than  standard  time  and  the  date  is  May  27,  the  total  correction 
will  be  20  minutes  plus  3  minutes.  The  sun  will  be  on  the  geo- 
graphic meridian  at  23  minutes  before  12:00  o'clock  standard 
time,  or  1 1 137  A.M.  The  shadow  of  the  wind  vane  support 
cast  on  a  horizontal  surface  will  point  due  north  when  the  sun  is 
on  the  meridian. 

Various  devices  are  used  to  ascertain  wind  direction  in  times 
of  very  low  velocity.  A  thread  flown  at  the  end  of  a  stick 
fastened  near  the  wind  vane  will  often  enable  an  observer  to 
discover  the  direction  of  the  wind  when  other  evidence  is 
absent.  An  ascending  smoke  column  is  swayed  by  a  breeze 
too  light  to  move  a  thread.  The  human  face  is  exceedingly 
sensitive  to  the  wind.  The  small  boy  who  wets  the  ball  of  his 
forefinger  and  holds  it  against  the  air  is  using  a  method  as  old 
as  the  voyage  of  Jason  in  search  of  the  Golden  Fleece.  Rather 
more  uncertain  is  the  movement  of  foliage.  In  many  instances 
a  wind  vane  whittled  from  a  very  thin  strip  of  wood  and  per- 
forated so  as  to  whirl  on  a  pin  as  an  axis  has  been  pretty  nearly 
as  serviceable  as  an  expensive  vane.  Small  vanes  made  of  thin 
aluminium  sheet  metal,  spread-tail  in  pattern,  have  answered 
every  purpose  required  for  ascertaining  wind  direction. 

On  the  other  hand,  the  commercial  weather  vane  on  a  church 
steeple  or  a  flagstaff  may  be  an  uncertain  guide.  Years  of 
weathering  may  have  rusted  it  fast  to  the  spindle;  and  improper 
mounting  may  prevent  its  coming  up  to  the  wind  by  many 
degrees. 


VARIATION  OF  WIND  VELOCITY  237 

Obtaining  wind  direction  from  the  movement  of  the  clouds 
is  frequently  misleading  as  to  results.  Sometimes  it  happens 
that  the  surface  wind  blows  from  one  direction,  while  clouds 
move  toward  another.  If  the  clouds  are  low,  the  direction 
whence  they  come  is  most  accurately  obtained  by  facing  them 
and  then  turning  at  a  right  angle  to  check  the  observation. 

Cross-winds,  that  is  wind  currents  of  different  directions, 
are  more  common  than  is  generally  known.  Airmen  have  learned 
their  meaning  and  watch  sharply  for  them.  They  are  apt  to 
occur  before  and  after  a  storm.  At  such  times  they  practically 
mark  the  advancing  or  the  retreating  edge  of  a  cyclonic  move- 
ment. Billow  clouds  are  the  earmarks  of  cross-winds  and  such 
cross-winds  are  usually  at  a  considerable  height.  Cross-winds 
are  very  common  along  the  coasts  of  large  bodies  of  water  where 
the  land  and  the  sea  breeze  alternate.  These  alternating  winds 
are  shallow,  however,  and  the  airman  usually  finds  the  steady 
prevailing  wind  at  an  altitude  of  half  a  mile  or  more.  The 
alternating  mountain  valley  winds  are  cross-winds  of  similar 
character. 

Cross-winds  are  not  always  discernible  to  the  airman  or  to 
the  marine  pilot.  They  become  visible  as  to  position  only  when 
difference  in  temperature  and  humidity  of  the  two  layers  pro- 
duces cloudiness  at  the  interface. 

Wind  Velocity. — The  velocity  of  the  wind  at  any  locality 
varies  greatly.  The  dead  calm  of  tropical  seas  is  frequently 
followed  by  hurricane  winds  having  a  velocity  exceeding  100  miles 
an  hour.  The  hurricane  that  wrecked  Galveston  blew  with  a 
velocity  estimated  at  more  than  125  miles  an  hour.  At  Cape 
Mendocino,  California,  a  velocity  of  144  miles  was  registered, 
and  at  Mount  Washington  a  mean  hourly  velocity  of  in  miles 
per  hour  was  registered  for  a  whole  day.1  At  Battery  Park, 
New  York  city,  the  anemometer  has  registered  a  velocity  of 
96  miles;  and  storm  winds  along  the  coast  have  reached  a 
velocity  of  100  miles  a  dozen  times  or  more.  Some  of  the 
strongest  winds  along  the  Atlantic  Coast  of  the  United  States 
are  storm  winds  of  a  recurved  part  of  West  Indian  hurricanes. 

For  the  greater  part,  the  mean  hourly  velocity  of  the  wind 
at  the  various  stations  provided  with  anemometers  varies  from 

1  February  27,  1886.  In  January,  1878,  a  velocity  of  186  miles  was  re- 
corded. 


238  THE   MEASUREMENTS   OF   WIND   VELOCITY 

5  miles  to  15  miles  per  hour.  On  sea  and  lake  coasts  it  is  materi- 
ally higher;  and  in  a  few  mountain  valleys  it  is  lower  than  3 
miles. 

Throughout  the  prairie  region  and  the  great  plains,  the  wind 
is  apt  to  be  steady,  its  velocity  varying  but  little  during  the 
daylight  period.  In  regions  where  land  breezes  alternate  with 
those  from  the  sea,  a  short  period  of  calm  precedes  each  change. 


Dial  of  the  Robinson  anemometer. 

Each  Weather  Bureau  station  is  provided  with  the  stand- 
ard anemometer  of  the  Robinson  pattern;  most  stations  are 
equipped  with  the  Friez  triple-magnet  register,  which  records 
both  direction  and  velocity  of  the  wind.  Recording  instruments 
of  this  character  are  used  at  the  principal  military  and  naval 
stations. 

The  cooperative  observation  sub-stations,  outnumbering  the 
regular  stations  about  twenty  to  one,  are  not  equipped  with 
recording  instruments  except  as  they  are  procured  at  private 
expense.  For  all  observers — volunteer,  regular  and  cooperative 


THE  ROBINSON  ANEMOMETER  239 

—the  Beaufort  wind  scale  J  affords  a  very  good  and  practical 
method  of  approximate  determination  of  wind  velocity.  The 
force  numbers  of  the  scale  adopted  by  the  Weather  Bureau 
are  the  same  as  those  of  the  British  scale;  the  velocity  in  miles 
per  hour  corresponding  to  the  force  numbers  differs  considerably. 

The  table,  p.  240,  gives  the  Beaufort  number,  designation 
of  wind  and  velocity  as  adopted  by  the  Weather  Bureau.  The 
physical  effects  are  those  of  the  British  scale. 

Cooperative  and  volunteer  observers  report  merely  the  pre- 
vailing direction  of  the  surface  winds,  except  as  specifically  di- 
rected. At  the  regular  Weather  Bureau  stations  the  direction 
and  velocity  of  upper  winds  are  noted,  a  necessary  step  for  the 
information  of  the  rapidly  growing  air  service.  More  informa- 
tion concerning  the  times  of  the  daily  maxima  and  minima  of 
wind  velocity  is  needed  for  the  benefit  of  air  service,  and  vol- 
unteer observers  may  be  very  helpful  in  obtaining  this  informa- 
tion. 

The  Anemometer. — For  all  ordinary  purposes  in  the  measure- 
ment of  wind  velocity  the  Robinson  anemometer  is  almost 
universally  used.  For  determining  the  mean  velocity  of  the 
wind  it  is  the  best  instrument  in  meteorological  service.  In- 
asmuch as  it  fails  to  record  momentary  gusts  of  wind  perfectly, 
a  Dines  pressure  anemometer  is  usually  added  to  the  equipment 
of  regular  stations.  A  wind  meter  of  the  Biram  type  is  also 
useful  in  the  measurement  of  wind  gusts;  it  merely  registers 
wind  velocities  without  recording  them. 

The  Robinson  type  of  anemometer  is  simple  in  construction 
and  does  not  easily  get  out  of  order.  Four  hemispherical  cups 
fastened  to  arms  6.72  inches  from  axis  to  center,  made  fast  to 
a  spindle,  communicate  their  motion  to  the  measuring  mechan- 
ism. The  upper  end  of  the  spindle  revolves  in  a  sleeve;  the 
lower  end  rests  in  an  oil  cup  which  also  is  a  bearing.  A  worm 
screw  thread  near  the  lower  end  engages  a  train  of  wheels. 
Two  of  these  are  registering  disks  turning  on  the  same  axis. 

^he  scale  was  devised  by  Admiral  Beaufort  in  1805,  chiefly  to  advise 
sailing-masters  of  the  kind  and  spread  of  sail  which  ships  of  the  line  might 
carry  and  their  probable  speed  under  such  sail.  Subsequently  it  was  ad- 
dressed to  fishing  smacks  and  trawlers.  More  recently  it  was  revised  for 
the  benefit  of  weather  observers.  A  few  meteorologists  have  used  it;  many 
observers  regard  it  as  too  complicated  to  be  of  practical  use.  Most  observers 
express  their  estimate  of  wind  velocity  in  very  few  terms:  as,  breeze,  light 
wind,  strong  wind,  and  gale. 


240 


THE   MEASUREMENTS   OF   WIND   VELOCITY 


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THE  ROBINSON  ANEMOMETER  241 

One  of  these  wheels  contains  100  teeth  and  the  other  99.  By 
means  of  this  differential  motion  the  registration  of  the  number 
of  miles  up  to  990  may  be  read  directly. 

A  type  of  Robinson  anemometer  in  which  registration  is 
made  with  index  hands  is  also  made.  Its  mechanism  does 
not  differ  otherwise  from  the  ordinary  type.  One  of  the  train 
of  geared  wheels  in  the  ordinary  form  of  anemometer  is  cut  with 
50  teeth.  If  replaced  with  one  cut  with  62  teeth  the  disks  will 
record  kilometers  instead  of  miles.  If  an  interchangeable  gear 
of  this  sort  is  desirable,  it  is  better  to  have  the  necessary  ad- 
justments made  by  the  manufacturer. 

Several  types  of  magnet  recorders  are  used  with  the  Robin- 
son anemometer.  In  the  type  most  commonly  used,  an  arm 
carrying  the  recording  pen  is  attached  to  the  armature  of  the 
magnet.  The  revolving  disk  carries  the  studs  representing  miles 
of. wind  movement  against  the  spring  which  acts  as  a  circuit 
closer;  the  recording  pen  thereupon  makes  an  offset  from  the 
straight  line  on  the  record  sheet.  Each  offset  represents  theo- 
retically a  mile  of  wind.  As  soon  as  the  stud  passes  the  contact 
spring — a  matter  of  from  five  to  ten  seconds — -the  pen  is  drawn 
back  to  its  normal  position.  The  fourth  and  fifth  studs  are 
bridged  and  the  closed  circuit  makes  the  offset  which  covers  a 
theoretical  mile  of  wind  movement.  This  is  a  convenience 
which  enables  the  number  of  miles  to  be  counted  in  groups  of 
ten.  Should  any  intermediate  mile-stud  fail  to  record,  the  failure 
will  not  be  lost;  it  is  included  in  the  count. 

Caution  is  necessary  at  times  in  reading  the  closed  circuit 
mile,  especially  when  it  represents  a  high  velocity  whose  measure 
is  to  be  determined  closely.  The  observer  must  choose  between 
closing  and  closing,  or  between  opening  and  opening  of  the 
circuit.  Because  the  opening — the  "breaking" — of  the  circuit 
is  quick  and  positive,  this  interval  is  considered  preferable  in 
determinations. 

The  record  sheet  is  necessary  in  finding  the  time  of  maximum 
or  of  minimum  velocity  of  the  wind.  From  it  one  may  also 
find  the  velocity  for  any  hour  of  the  day.  And  inasmuch  as 
such  information  is  frequently  required  in  suits  at  court,  accurate 
dating  and  time-checking  of  the  record  sheets  are  essential. 

The  totals  for  any  specific  time  may  be  read  from  the  record 
sheets.  Daily  and  monthly  totals  may  be  read  from  the  dial. 


242 


THE   MEASUREMENT   OF   WIND   VELOCITY 


Indeed,  many  observers  prefer  to  make  dial  readings,  except  in 
cases  where  hour  totals  are  required. 

The  establishment  of  the  time  of  the  daily  maxima  and  the 
daily  minima  is  a  most  useful  problem  for  cooperative  and 
volunteer  observers  to  undertake.  This  is  very  easily  solved 
from  the  daily  record  sheets,  but  close  observation  will  enable 
an  observer  to  get  pretty  accurate  results  without  the  aid  of 
instruments.  Daily  observations  may  be  summarized  in  monthly 


Wind  velocity  recording  apparatus. 

averages,  and  from  these  the  seasonal  averages  may  be 
determined. 

To  obviate  the  inconvenience  of  changing  record  sheets  at 
midnight,  they  are  most  commonly  changed  at  noon.  A  day's 
record  consists  of  the  two  lower  lines  of  one  sheet  and  the  two 
upper  lines  of  the  sheet  for  the  day  following.  The  aggregates 
may  be  kept  in  half-day  totals,  but  it  is  better  to  carry  them 
over  and  enter  them  on  the  record  sheet  of  the  following  day. 
The  necessary  thing  is  a  definite  plan  followed  with  intelligence 
rather  than  slavish  exactness. 

Changing  anemograph  sheets  precisely  at  noon  is  highly 
desirable.  If  the  clock  is  either  fast  or  slow,  its  rate  is  best 


CARE  OF  ANEMOMETER  243 

established  at  that  time.  The  pressure  of  the  pen  on  the  sheet 
is  bound  to  vary  slightly  from  day  to  day,  and  this  in  itself  is 
likely  to  cause  the  rate  of  the  clock  to  change.  The  pen  should 
touch  the  paper  positively  but  lightly. 

The  pen  requires  frequent  cleaning;  whenever  the  nib 
appears  clogged  it  should  be  wiped  with  a  bit  of  soft  rag.  Half 
a  dozen  times  a  year  it  should  be  removed  and  made  thoroughly 
clean,  scraping  off  the  sediment  that  washing  will  not  remove. 
A  clean  pen  and  good  ink  will  leave  a  record  as  clear  and  clean 
as  though  the  lines  had  been  made  with  a  drawing  pen. 

The  wiring  plan  for  the  triple  register  is  set  forth  in  detail 
in  Circular  D,  Instrument  Division,  and  the  details  need  not 
be  rehearsed  here.  Wherever  wires  pass  through  woodwork, 
porcelain  tube  insulators  are  required  by  insurance  regulations. 
If  the  wiring  is  situated  where  contact  with  electric  light  wires 
is  possible,  heavily  insulated  wires,  such  as  are  prescribed  by 
local  regulations,  should  be  used.  All  permanent  wire  joints 
and  splices  should  be  snugly  twisted  a  length  of  2  inches — or, 
better,  soldered — and  wrapped  with  tape.  All  outside  wires 
should  be  held  by  insulators. 

Batteries. — In  operating  a  triple  register,  10  cells  of  battery 
are  required ;  for  the  wind-direction  register,  4  cells ;  for  the 
anemometer,  3  cells;  for  the  sunshine  and  rainfall  recorder, 
3  cells  in  common.  For  a  -2-magnet  register,  recording  wind- 
velocity  and  sunshine,  3  cells  in  common  are  sufficient.  Unless 
the  electromotive  force  is  strong,  however,  the  sunshine  recorder 
may  fail  to  register  when  the  anemometer  contact  is  on  the 
bridge.  This,  however,  need  not  lead  to  error;  the  bridge 
contact  is  always  a  long  offset  on  the  anemograph. 

The  requirement  of  battery  cells  for  magnet  registers  is 
steadiness  rather  than  strength.  Dry  cells  run  down  in  electro- 
motive force  so  quickly  that,  if  the  wind  is  on  the  bridge  for 
more  than  a  few  minutes,  the  sunshine  recorder  fails  to  register 
unless  operated  by  a  separate  battery.  The  ordinary  "wet 
cells"  are  not  much  better. 

When  storage  batteries  suitable  for  the  work  are  not  available, 
the  cells  of  the  Edison  primary  type  are  the  best.  Such  a  cell 
properly  charged  gives  a  feeble  but  steady  current  on  a  closed 
circuit  for  nearly  400  hours.  It  will  operate  a  2-magnet  register 
for  more  than  a  year. 


CHAPTER  XXI 

THE  MEASUREMENT  OF  SUNSHINE:  SUNSHINE 
RECORDERS 

Recording  the  daily  duration  of  sunshine  is  a  part  of  the 
work  of  all  Weather  Bureau  stations;  it  is  carried  on  also  at 
agricultural  experiment  stations,  at  university  laboratories,  and 
at  many  aviation  fields.  The  objective  information  may  be 
different  in  the  various  cases;  the  methods  of  measurement  are 
usually  the  same. 

Sunshine  measurements  are  not  required  of  cooperative 
observers;  their  reports,  however,  include  an  estimate  of  cloudi- 
ness; less  than  one-third  of  cloudiness,  clear;  one-third  to  two- 
thirds,  partly  cloudy;  and  two^thirds  or  more,  cloudy.  A  day 
with  a  sky  full  of  broken  clouds  is  necessarily  recorded  as  cloudy ; 
nevertheless  the  registered  sunshine  may  be  almost  continuous. 
At  times  the  cloud  film  of  a  sky  completely  overcast  may  be 
so  thin  that  a  sunshine  recorder  of  any  sort  will  register  a  con- 
siderable part  of  the  day.  On  the  other  hand,  a  light  dust 
haze  may  interfere  materially  with  registration,  although  to  the 
sense  of  sight,  the  light  seems  normal.  The  amount  of  sun- 
shine, therefore,  cannot  be  reckoned  from  a  record  of  cloudiness. 
Even  with  recording  instruments  having  the  best  possible  ad- 
justments, the  record  of  sunshine  for  a  given  period  is  an  ap- 
proximate only ;  there  is  no  such  refinement  in  sunshine  measure- 
ments as  exists  in  measurements  of  pressure  or  of  temperature. 

Sunshine  Recording  Instruments. — The  various  sunshine 
recording  instruments  may  be  reduced  to  three  types:  the 
burning-glass  type,  such  as  the  Campbell-Stokes  recorder; 
the  photographic  type,  such  as  the  Jordan  recorder,  for  many 
years  used  in  the  United  States  Weather  Bureau;  and  the 
thermo-electric  type;  of  which  the  Marvin  recorder  is  practi- 
cally the  only  one. 

244 


SUNSHINE  RECORDERS 


245 


The  Campbell-Stokes  recorder  consists  of  a  sphere  of  colorless 
or  slightly  yellow  glass  of  a  high  degree  of  transparency.  It  is 
mounted  in  a  frame  in  front  of  a  concave  surface  set  at  focal 
distance  from  the  glass  sphere.1  The  central  line  of  the  record- 
ing chart  must  lie  in  the  plane  of  the  true  meridian.  The  frame 
itself  is  adjustable  to  the  sun's  altitude.  The  recording  chart  is 
graduated  to  hour  intervals.  The  focal  rays,  shifting  with  the 
position  of  the  sun,  char  a  line  along  the  chart.  The  charred 
line  represents  the  duration  of  the  sunshine.  When  the  sunshine 
has  occurred  at  short  and  irregular  intervals  the  aggregate 
duaration  may  be  found  most  quickly  by  placing  the  edge  of  a 


Campbell-Stokes  sunshine  recorder. 

sheet  of  paper  along  the  record  and  marking  thereon  lengths 
equal  to  the  lengths  of  the  successive  charrings.  By  sliding  the 
paper  along  the  trace,  the  lengths  form  a  continuous  line. 
Their  aggregate  then  may  be  measured  along  the  graduations 
of  the  paper. 

1  A  sphere  of  the  sort  has  some  of  the  characteristics  of  a  prism;  it  refracts 
the  various  components  of  a  ray  of  light  unequally — red  rays  the  least,  violet 
rays  the  most.  The  registering  paper  gives  the  best  record,  on  the  whole, 
when  set  at  the  focus  of  greatest  light  intensity.  The  best  focal  distance 
cannot  always  be  determined  by  rule  of  thumb,  however;  it  varies  slightly 
with  the  locality,  and  therefore  the  optimum  focal  distance  must  be  determined 
by  the  observer. 


246 


THE   MEASUREMENT  OF  SUNSHINE 


The  intensity  of  the  charring  varies  according  to  the  condi- 
tion of  the  air  as  to  cleanliness  and  freedom  from  moisture.  If 
the  air  is  clear  and  dry,  the  focal  rays  burn  a  long  gap  in  the 
sheet.  On  the  other  hand,  a  moist  or  a  polluted  air  may  absorb 


Sunshine  differential  thermometer,  electrically  connected  with  the  recording 

apparatus. 

so  much  heat  that  the  surface  of  the  recording  chart  is  merely 
discolored.  The  record  for  the  first  half  hour  or  more  after 
sunrise  is  usually  indistinct;  this  is  usually  true  of  the  time  just 
before  sunset.  If  a  cloud  obscures  the  sun,  even  for  a  few  minutes, 
the  burning  process  is  arrested. 

The  charts  used  in  this  type  of  sunshine  recorder  may  be 
changed  at  any  time  between  sunset  and  sunrise  of  the  following 


MARVIN  SUNSHINE  RECORDER  247 

day.  Each  chart,  therefore,  carries  the  record  of  a  full  daily 
period  of  sunshine. 

The  Campbell-Stokes  recorder  is  simple  in  construction  and 
inexpensive.  It  requires  neither  clockwork  nor  electrical 
mechanism  in  its  operation.  The  records  are  unsightly,  but  they 
are  ineffaceable  and  permanent.  As  a  piece  of  mechanism  it  is 
practically  fool-proof.  In  the  laboratories  of  Europe  its  use 
is  general. 

Photographic  recorders  depend  on  the  action  of  sunlight  on 
sensitive  paper.  The  record  sheet  is  placed  within  a  camera 
of  circular  section.  A  minute  aperture  permits  a  spot  of  light 
to  enter  the  camera  and  fall  on  the  record  sheet.  In  one  form 
there  are  two  apertures,  one  for  the  period  from  sunrise  till 
noon,  the  other  from  noon  to  sunset.  The  changing  position  of 
the  sun  causes  the  spot  of  light  to  traverse  the  record  sheet  in 
an  opposite  direction.  After  exposure  the  sheets  are  developed 
and  fixed  by  ordinary  photographic  processes.  Silver  paper  gives 
the  most  legible  charts;  and  when  a  bit  of  blue  glass  is  used  as 
a  light  filter,  the  line  of  record  is  more  sharply  drawn  and 
clearer.  Silver  paper  is  expensive,  however,  and  ordinary  blue- 
print paper  is  more  commonly  used. 

The  time  and  effort  required  to  prepare  the  sensitive  paper, 
and  to  develop  and  fix  the  record  sheets  is  the  great  objection 
to  photographic  recorders.  In  some  respects  they  are  more 
accurate  in  time  measurement  than  any  other  form;  and  in  this 
particular  they  have  possibilities  not  possessed  by  any  other 
recorders. 

The  Marvin  thermo-electric  recorder  is  used  at  United  States 
Weather  Bureau  stations  and  in  most  meteorological  labora- 
tories. It  consists  of  a  differential  thermometer  in  a  vacuum 
tube  and  a  recording  apparatus.  The  expansion  of  a  volume  of 
air  in  a  blackened  tube  pushes  a  column  of  mercury  between 
two  platinum  points,  the  ends  of  which  pierce  the  tube,  thereby 
making  an  electric  circuit  possible  in  the  recording  apparatus. 
The  air  volume  within  the  blackened  tube  is  exceeding  sensi- 
tive to  heat.  Even  in  the  coldest  weather,  the  heat  of  direct 
sunshine  is  sufficient  to  push  the  mercury  to  the  circuit-making 
points;  with  the  absence  of  sunshine  the  mercury  drops  below 
them.  Inclining  the  tube  takes  some  of  the  weight  off  the  air 
chamber  and  causes  the  mercury  to  be  more  easily  lifted.  In 


THE   MEASUREMENT   OF   SUNSHINE 


MARVIN  SUNSHINE  RECORDER  249 

order  to  overcome  the  friction  of  the  mercury  against  the  glass, 
it  is  lubricated  with  alcohol.  The  upper  part  of  the  tube,  which 
is  not  blackened,  .also  contains  air.  A  dextrous  shaking  of  the 
tube  will  transfer  bubbles  of  air  from  one  end  to  the  other, 
thereby  holding  the  column  of  mercury  to  any  desired  height, 
or  to  any  desired  distance  from  the  circuit  points. 

The  stand  should  be  mounted  in  a  locality  that  is  not  shaded. 
The  tube  should  be  in  the  plane  of  the  true  meridian,  and  face 
the  south.  The  angle  of  inclination  should  be  roughly  about  45 
degrees.  In  summer  it  should  be  a  little  nearer  to  the  vertical; 
in  winter  a  little  more  to  the  horizontal.  The  angle  depends 
partly  on  the  amount  of  air  in  the  lower  bulb  and  partly  on  the 
position  that  gives  it  the  maximum  of  insolation.  In  general, 
the  results  are  best  when  the  top  of  the  mercury  is  from  half 
an  inch  to  an  inch  above  the  circuit  points  during  the  warmest 
part  of  the  day. 

The  recording  device  is  attached  to  that  of  the  anemometer, 
using  the  same  sheet  but  a  different  recording  pen.  When 
the  sun  is  shining,  a  contact  of  the  clock  completes  the 
circuit.  The  movement  of  the  armature  operates  a  jigger, 
which  moves  the  pen  once  every  minute.  The  jigger  and  the 
progressive  movement  of  the  drum  cause  the  pen  to  make  steps 
in  series  of  five,  back  and  forth.  Each  series  represents  five 
minutes  of  time.  These  continue  while  the  sun  is  shining.  When 
the  sun  is  not  shining,  the  mercury  in  the  thermometer  drops 
away  from  the  platinum  circuit  points.  The  pen  then  draws 
a  straight  line. 

.Even  when  carefully  adjusted,  the  recorder  will  not  begin  to 
register  for  some  time  after  sunrise;  it  ceases  to  register  a 
short  time  before  sunset.  These  periods  must  be  measured 
from  time  to  time  by  the  observer,  taking  the  time  of  sunrise 
and  sunset  from  a  reputable  almanac  for  the  approximate 
latitude  of  the  station.  These  intervals  are  the  twilight 
corrections. 

The  morning  twilight  correction  is  usually  somewhat  greater 
than  that  of  evening.  Both  vary  slightly  between  winter  and 
summer.  In  localities  where  city  smoke  and  floating  dust  do 
not  contribute  to  instrumental  sluggishness,  the  morning  cor- 
rection should  not  exceed  one  hour;  the  evening  correction 
should  not  be  more  than  half  as  much.  The  presence  of  smoke, 


250  THE   MEASUREMENT  OF   SUNSHINE 

dust  and  haze  may  extend  this  correction  to  more  than  one  and 
one-half  hours. 

Measurement  of  Sunshine. — The  adjustment  for  registration 
practised  by  the  Weather  Bureau  is  based  on  experience  and  is 
reasonable.  The  tube  holding  the  thermometer  should  be 
inclined  so  that  the  recorder  will  register  when  the  actual  disk 
of  the  sun — not  a  shapeless  blotch  of  light — can  be  discerned 
through  the  clouds  of  an  overcast  sky.  It  is  better  to  make  the 
adjustment  when  the  sun  is  about  two  hours  high,  by  inclining 
the  tube.  The  observer  must  wait  for  such  a  day,  and  perhaps 
several  trials  may  be  necessary. 

The  computation  of  the  total  actual  hours  of  sunshine  may 
be  made  by  any  system  which  the  observer  finds  convenient. 
If  the  total  sunshine  is  not  more  than  a  few  hours,  it  is  perhaps 
most  easily  counted  in  the  manner  suggested  in  a  previous 
paragraph — that  is,  measurement  along  the  edge  of  a  strip  of 
paper.  When  the  obscuration  by  cloudiness  is  slight,  or  is 
absent,  the  following  method  may  be  followed : 

From  the  total  possible  hours  for  the  day  deduct : 

(a)  The  excess,  if  any,  of  obscuration  over  the  twilight  corrections; 

(b)  The  total  of  obscurations  due  to  cloudiness;    thus: 
'Total  possible  hours 13:36 

Twilight  corrections .  .-. I  30 

Excess  over  twilight  corrections o  154 

Cloud  obscurations 2:22  4:46 


Actual  hours 8 150 

The  calculations  may  be  made  in  detail  on  the  back  of  the 
record  sheet.  On  the  face  of  the  sheet  these  should  be  entered 
in  the  proper  place: 

Total  hours  carried  forward 172 124 

Saturday  (or  current  day) .  .  .  .' 8:50 


Total  since  first  of  month 181:14 

By  this,  or  by  a  similar  method,  the  computation  for  the  month 
is  finished  on  the  last  day  of  the  month.  In  Weather  Bureau 
practise,  minutes  of  time  are  reckoned  in  decimals  of  an  hour, 
in  the  measurement  of  sunshine. 

Sunshine  records  are  approximate  only;    close  calculation, 
however,    will    bring    reasonably    accurate    results.      At    times 


CONCRETE  RESULTS  OF  SUNSHINE  251 

the  judgment  of  a  careful  observer  may  be  more  trustworthy 
than  an  imperfectly  adjusted  instrument.  At  times,  too,  there 
may  be  momentary  periods  of  sunshine  which  are  not  registered 
at  all.  Even  in  the  absence  of  all  measuring  instruments,  an 
observer  whose  record  consists  merely  of  the  total  of  overcast 
days  is  gathering  information  of  great  value. 

Concrete  Results  of  Sunshine  Records. — It  is  well  to  bear 
in  mind  that  the  mere  gathering  and  tabulating  of  monthly  sta- 
tistics of  sunshine  is  not  an  end,  but  merely  a  means  to  an  end. 
Knowledge  of  any  sort  possesses  but  little  value  unless  it  can 
be  applied  to  the  betterment  of  humanity.  In  agriculture  the 
results  may  be  applied  so  as  to  obtain  more  definite  knowledge 
concerning  the  growth  and  maturity  of  plants: — essentially  the 
minimum  amount  of  sunshine  necessary  to  fructification.  In 
almost  every  department  of  agriculture  the  total  of  sunshine 
has  a  direct  bearing  on  the  amount  of  evaporation. 

In  climatology,  much  more  information  concerning  the 
relation  of  sunshine  to  public  health  is  required.  The  healing 
value  of  sunlight  is  not  overestimated;  its  value  in  therapy  of 
the  mind  is  underestimated.  The  mental  depression  following 
prolonged  spells  of  overcast  skies  is  marked. 

In  commerce  and  transportation,  the  effects  of  obscuration 
on  visibility  is  becoming  a  matter  of  systematic  study  and  in- 
vestigation. The  impairment  of  visibility  costs  more  than 
money;  its  toll  of  human  life  is  heavy. 

In  military  and  naval  strategy,  helio-signaling  depends  on 
sunshine;  so  also,  sunshine  is  the  key  to  many  problems  involv- 
ing visibility. 

The  efforts  of  a  single  observer  may  not  solve  general  prob- 
lems, but  they  will  go  a  long  way  in  solving  the  specific  cases  of 
his  own  bailiwick.  The  observer  may  determine  whether  or 
not  specific  times  of  obscuration — daily  or  seasonal — prevail. 
In  many  parts  of  the  country  good  beginnings  have  been  made 
already  by  volunteer  observers.  Incidentally,  there  is  no 
station,  permanent  or  transient,  from  which  additional  informa- 
tion would  not  prove  of  value. 

The  area  in  which  the  per  cent  of  sunshine  is  less  than  40 
is  very  small,  and  but  little  of  it  is  crop-growing  land.  The 
region  of  greatest  sunshine,  for  the  greater  part,  is  deficient  in 
rainfall.  Irrigated  lands,  however,  produce  crops  that  are 


252  THE   MEASUREMENT  OF  SUNSHINE 

extraordinary  in  quantity  and  unsurpassed  in  quality.  The 
Lake  region  is  below  the  average  in  sunshine,  but  the  deficiency 
does  not  impair  the  quality  of  the  fruit  crop.  The  region  of 
greatest  cotton  production  receives  from  60  to  70  per  cent; 
and  a  comparison  of  this  region  with  that  of  a  lower  per  cent 
shows  that  the  higher  per  cent  is  essential.  Practically  every 
part  of  the  United  States  receives  enough  sunshine  for  a  fair 
crop  production,  and  a  very  great  part  receives  enough  for  maxi- 
mum production. 

In  the  latitude  of  New  Orleans,  June  days  are  about  14  hours 
long;  in  the  latitude  of  Minneapolis  they  are  nearly  16  hours. 
The  aggregate  warmth  is  about  the  same  in  each  case.  The 
oblique  rays  of  the  sun  and  their  lower  heating  power  are 
balanced  by  greater  duration  in  time. 


APPENDIX 


REFERENCE   TABLES 
C.   G.  S.   UNITS 

The  construction  and  evolution  of  the  system  of  electrical 
units  now  in  general  use  began  in  Europe  where  the  metric  sys- 
tem originated.  Two  of  the  basic  units,  the  centimeter  and  the 
gram,  are  metric  units;  the  second  of  time  is  universally 
employed  in  time  measurements.  * 

The  meter,  3.2808  feet  =  39. 37  inches,  is  theoretically  the 
one  ten-millionth  part  of  the  earth's  quadrant;  actually  it  is 
the  length  of  a  metal  rod  differing  from  the  theoretical  value 
by  about  three-fourths  of  a  millimeter.  The  prefixed  multiples, 
deka-,  hecto-,  kilo-  and  mega-  are  in  ten-fold  ratio.  They  indi- 
cate respectively  10,  100,  1000  and  1,000,000.  The  prefixed 
decimal  divisions,  deci-,  centi-,  milli-  and  micro-  indicate  tenths, 
hundredths,  thousandths  and  millionths. 

The  C.  G.  S.  units  are  generally  used  in  the  United  States  in 
chemical  and  in  abstract  physical  determinations.  On  account 
of  their  inconvenient  magnitudes  their  use  has  not  extended  to 
mechanics;  and  for  this  reason  the  employment  of  them  is 
strenuously  resisted  by  manufacturers  in  every  line. 

Units  of  the  C.  G.  S.  System.— The  centimeter,  the  one- 
hundredth  part  of  the  meter,  is  the  unit  of  length.  I  cm  = 
0.0328  foot. 

The  gram,  the  unit  of  mass,  is  the  weight  of  I  cu  cm  of  pure 
water  under  standard  conditions.  I  g  =  0.0022  pound  =  15.432 
grains.  The  weight  of  a  cubic  centimeter  of  a  substance  also 
represents  its  specific  gravity. 

The  square  centimeter  is  the  unit  of  area.  I  sq  cm  or  cm2  = 
0.001076  sq  ft  =  0.1550  sq  in. 

253 


254  APPENDIX 

The  cubic  centimeter  is  the  unit  of  volume.  I  00  =  0.0000353 
cu  ft  =  o.o6i  cu  in. 

The  unit  of  density  is  numerically  the  same  as  the  specific 
gravity.  It  is  the  rate  of  I  gram  per  cubic  centimeter. 

The  unit  of  velocity  is  the  velocity  of  I  centimeter  per  second ; 
I  cm  per  second  =  0.0328  foot  per  second  =  0.0224  m.  per  hour. 

The  unit  of  acceleration  is  the  rate  of  a  unit  of  velocity  per 
second  or  I  centimeter  per  second,  per  second. 

The  unit  of  force,  I  dyne,  is  the  force  which  imparts  to  I. 
gram  a  velocity  of  I  centimeter  per  second,  per  second.  It  is 
sometimes  expressed  in  the  term  poundals;  I  dyne  =  0.0000722 
poundal.  The  acceleration  of  gravity  oif  a  falling  body  varies 
in  different  latitudes  because  the  earth  is  spheroidal  and  not 
globular  in  form.  In  latitude  45°,  the  latitude  to  which  results 
commonly  are  reduced,  the  acceleration  is  980.621  centimeters 
per  second,  per  second.  If  the  relative  value  at  latitude  45° 
be  taken  as  a  unit,  the  relative  value  at  the  equator  will  be 
0.9974;  at  the  pole  1.0027. 

The  unit  of  pressure  is  I  dyne  per  square  centimeter.  The 
megadyne  is  a  more  convenient  unit  and  generally  is  used. 
The  megadyne  is  commonly  called  I  bar.  Barometric  pressure 
is  read  in  kilobars  and  its  subdivisions  in  millibars.  I  bar  = 
1000  millibars  =  0.00 1  kilobar.  I  kilobar  =  0.03386  inch; 
I  inch  =  33. 864  millibars;  I  millimeter  =  1.333  millibars.  The 
barometric  pressure  at  29.53  inches  =1000  millibars,  which  is 
equivalent  to  an  altitude  of  338  feet  above  mean  sea  level. 
The  dyne  may  be  conceived  as  the  pressure  upon  the  hand — 
that  is,  the  weight — of  a  piece  of  very  thin  tissue  paper  I  centi- 
meter square. 

The  unit  of  rainfall  is  I  millimeter.  For  all  practical  purposes, 
the  reduction  of  hundredths  of  an  inch  in  rainfall  to  millimeters 
of  rainfall  is  effected  by  moving  the  decimal  point  two  places 
to  the  right  and  dividing  by  4.  Thus,  if  the  catchment  of  a 
storm  is  2.40  inches;  the  division  gives  60  millimeters.  A  more 
accurate  result  will  be  obtained  by  using  3.94  as  the  divisor. 
Multiplying  by  .04  reduces  millimeters  of  rainfall  to  inches  of 
rainfall. 

The  practical  unit  of  wind  velocity  is  I  meter  per  second — 
that  is,  100  times  the  C.  G.  S.  unit. 

The  practical  unit  of  wind  force  is  I  kilodyne  per  unit  of  area. 


REFERENCE  TABLES  255 

The  practical  unit  of  wind  pressure  is  the  millibar.  Wind 
force  is  not  always  exactly  proportional  to  the  area. 

The  practical  unit  of  radiation  is  the  gram  calorie  (in  distinc- 
tion from  the  great  calorie)  or  the  warmth  required  to  raise  I 
gram  of  water  i°  C  in  temperature. 

The  erg,  the  unit  of  work,  is  the  amount  of  work  done  when  a 
mass  of  I  gram  moves  a  distance  of  I  centimeter,  against  a 
resistance  of  I  dyne.  A  more  practical  unit  is  the  joule,  or 
10,000,000  ergs. 

The  unit  of  time  is  the  second,  of  which  the  mean  solar  day 
contains  86,400. 

Units  of  Magnetism  and  Electricity. — Magnetism  may  be 
either  positive  or  negative.  Bodies  charged  with  the  same  kind 
of  magnetism  repel  each  other;  charged  with  opposite  kinds, 
they  attract.  Commercial  magnets  are  usually  marked,  the 
letter  N  or  a  dash  being  stamped  upon  the  north-seeking  pole. 
The  convenient  unit  of  magnetism  is  one  which  attracts  or 
repels  an  equal  quantity  at  a  distance  of  I  centimeter. 

The  absolute  unit  of  current  flowing  through  a  centimeter 
of  wire  acts  with  a  force  of  I  dyne  on  a  unit  of  magnetism  I 
centimeter  distant  from  every  point  of  the  wire. 

An  ampere,  the  practical  unit  of  current,  is  the  electro- 
motive force  of  i  volt  against  a  resistance  of  I  ohm.  It  is  the 
tenth  part  of  the  absolute  unit.  An  ordinary  dry  cell  gives 
a  current  of  about  2  amperes,  a  Daniels  wet  cell,  I  ampere. 
Dry  cells  differ  somewhat  in  strength. 

A  volt,  conversely,  is  the  electromotive  force,  or  "  electric 
pressure,"  which,  flowing  in  a  conductor  having  a  resistance 
of  I  ohm,  will  yield  I  ampere  of  current.  The  ordinary  dry 
cell,  when  fresh,  has  an  electromotive  force  of  about  2 
volts. 

The  ohm  is  the  unit  of  resistance.  For  all  practical  purposes 
it  is  the  resistance  of  50  meters  of  copper  wire  I  millimeter  in 
diameter.  Theoretically  it  was  intended  to  be  the  resistance  of 
a  wire  in  which  I  ampere  of  current  in  I  second  would  generate 
the  amount  of  heat  equivalent  to  10,000,000  ergs. 

The  watt  is  the  unit  of  power.  At  the  rate  of  10,000,000  ergs 
per  second,  a  current  of  I  ampere  having  the  pressure  of  I  volt 
has  a  value  of  I  watt.  A  common  candle  has  the  heating  equiva- 
lent of  about  60  watts.  One  horse  power,  the  power  required 


256  APPENDIX 

to  lift  33,000  pounds  I  foot  high  in  a  minute  of  time,  is  rated 
about  746  watts. 

A  coulomb  is  the  unit  of  quantity.  It  is  the  quantity  of  elec- 
tricity transferred  by  a  current  of  I  ampere  in  one  second. 

The  Jarad  is  the  unit  of  capacity.  The  capacity  of  a  con- 
denser is  theoretically  the  electricity  that  can  be  stored  in  it 
by  a  cell  of  known  electromotive  force.  For  purposes  of  exact 
measurement  the  cell  should  have  an  electromotive  force  equal 
to  i  unit  absolute  measure.  It  is  practically  a  condenser  which, 
charged  with  I  coulomb  of  current,  has  a  difference  of  potential 
of  i  volt. 

CHEMICAL  FORMULAS 

Freezing  Mixtures. — Salt,  i  part;  snow  at  32°,  2  parts,  pro- 
duce a  zero  mixture. 

Calcium  chloride,  2  parts;  snow  at  32°,  I  part,  produce  a 
temperature  of  —40°  F. 

Potassium  hydrate  (concentrated)  at  32°  and  snow  at  32°, 
equal  parts,  produce  a  temperature  of  —30°  F. 

Ammonium  chloride,  potassium  nitrate  and  water  at  32°, 
equal  parts,  produce  a  temperature  of  — 10°  F. 

The  temperature  produced  by  these  mixtures  is  approxi- 
mate only.  The  first  mentioned  may  vary  not  more  than  3 
degrees;  usually  the  variation  is  not  more  than  I  degree;  a 
variation  of  as  much  as  10  degrees  may  occur  in  the  second.  As 
a  rule,  the  larger  the  volume  of  the  mixture  the  better  the  result. 

Chemical  Hygroscopes. — Various  chemical  salts  are  sensi- 
tive to  the  moisture  of  the  air;  some  of  them  change  color  with 
the  absorption  or  the  discharge  of  moisture.  Cobaltic  chloride 
is  the  basis  of  most  of  the  commercial  hygroscopes  of  this  char- 
acter. A  solution  consisting  of  5  grains  of  cobaltic  chloride, 
50  grains  of  gelatine  and  I  fluid  ounce  of  water  is  a  good  por- 
portion.  There  must  be  a  complete  emulsification  of  the  gela- 
tine. A  strip  of  unsized  paper  wet  with  the  emulsion  is  normally 
pink  in  moist  air,  violet  in  moderately  moist  air,  and  blue  in  dry 
air.  When  the  winter  fires  are  on  in  dwellings,  the  strip  of 
paper  is  persistently  blue.  Ordinarily  it  does  not  begin  to  turn 
until  the  moisture  of  the  air  is  about  75  per  cent. 

Ozone  Test  Papers. — Qualitative  tests  for  ozone  are  often 
desirable.  The  following  solution  is  highly  regarded.  Distilled 


REFERENCE   TABLES  257 

water,  I  oz;  starch,  25  grains;  potassium  iodide,  4  grains. 
Dissolve  the  potassium  iodide  in  a  small  part  of  the  water; 
boil  the  starch  in  the  remaining  part ;  mix  and  shake  thoroughly. 
Moisten  strips  of  unsized  white  paper  and  suspend  them  in  the 
thermometer  shelter  or  in  shaded  open  air.  Usually  an  exposure 
of  from  3  hours  to  10  hours  is  required.  The  ozone  decomposes 
the  potassium  iodide,  thereby  turning  the  paper  blue  in  color. 
This  is  probably  the  best  test. 

A  slip  of  paper  moistened  with  a  solution  of  manganous 
sulphate  is  turned  brown  by  ozone.  The  reaction  oxidizes  the 
manganous  to  manganic  sulphate. 

A  paper  strip  smeared  thinly  with  lead  sulphide  is  more  or 
less  bleached  by  ozone,  the  sulphide  being  oxidized  to  a  sulphate, 
which  is  white.  This  test  is  characteristic  if  the  sulphide  is 
smeared  upon  black  paper. 

Storm  Glasses. — The  so-called  "  storm-glass  "  which  is  sold 
under  several  fanciful  names,  consists  of  a  solution  just  beyond 
the  point  of  precipitation  inclosed  in  a  thin  glass  tube  about  7 
inches  long  and  half  an  inch  in  diameter.  The  solution  consists 
of  camphor,  10  parts;  potassium  nitrate  (saltpeter)  5  parts; 
ammonium  chloride  (sal  ammoniac)  5  parts;  95  per  cent 
alcohol  105  parts;  and  distilled  water  45  parts.  The  virtue  of 
the  solution  depends  on  the  fact  that  neither  the  solution  nor 
the  precipitation  is  complete;  the  proportion  of  alcohol  must 
be  regulated  to  prevent  more  than  a  small  part  of  the  chemical 
salts  from  precipitation.  The  glass  must  be  thin  enough  and 
elastic  enough  to  yield  slightly  to  changes  in  atmospheric 
pressure.  About  I  inch  of  air  space  in  the  tube  adds  to  the 
sensitiveness  of  precipitation.  An  increase  in  pressure  causes 
the  precipitation  to  extend  nearly  to  the  top  of  the  solution; 
a  decrease  causes  increased  solution  and  a  settling  to  the  bottom 
of  the  tube.  With  a  very  low  barometer  the  precipitated  matter 
sinks  to  the  bottom  and  becomes  gelatinous.  Increase  of  pres- 
sure is  followed  by  the  formation  of  minute  crystals  that  grow 
in  size.  The  only  weather  changes  indicated  are  those  foretold 
by  a  rising  or  by  a  falling  barometer. 


258 


APPENDIX 


TABLE  i — MISCELLANEOUS  EQUIVALENTS 


LINEAR  EQUIVALENTS 

i  mile  =     320  rods 
=    1760  yards 
=   5280  ft 
=  63360  in 
=          i  .60935  kilometers 

i  rod=     0.003125  mi 
=    16.5  ft. 
=  198.0  in 
=     5.0292  meters 

i  yard  =  o .  000568  mi 

=  0.9144      meter 


i  foot 


=  0.3333  yd 
=  0.000189  mi 
=  0.3048  meter 


i  inch  =  1000  mils 
=  0.08333  ft 
=  0.02777  yd 
=  2 . 54    centimeters 

i  mil  =0.001    in 

=  0.0254  millimeter 

i  fathom  =  6  ft 

=  i .  8288  meter 


i  chain  (engineers) 
=  100  ft 
=  6 . 0606  rods 
=  !  •  5 1 57  Gunter's  chain 
=  30.48  meters 

I  Gunter's    chain   =66  ft 
=  4  rods 
=  o .  66  chain 
=  0.0125  mi 
=  20. 1 1 68  meters 

I  link  =  7. 92  in 

i  kilometer  =  0.62 1 37  mi 

=  198 . 8384  rods 
=  1093.61  yds 
=  3280.83  ft 

I  meter  =  39. 37  in 
=  3.28083  ft 
= i .09361  yds 

I  centimeter  =  0.393 7  in 
=  0.032808  ft 

I  millimeter  =.0.03937  in 

=  39-37  mils 

=  1000  mikrons 
I  mikron=o.ooi  mm 


i  nautical  mile  =  6080. 2 7  ft  US 

=  i.  1516  stat  mi  US 
=  6080  ft  Br  Ad 
=  1.1515  stat  mi 

i  statute  mile  =  0.8684  naut  m^  US 
i  geographical  mile=o°i'  at  equator 
=  6087.15  ft 
=  i .  1528  stat  mi 

i  nautical  mi  Br  Ad 

=  1853.248  meters 
i  geographical  mi  (U  S) 

=  1855.345  meters 


SQUARE  EQUIVALENTS 

I  square  mile  =  640  acres 

=  102400  sq  rd 
=  3,097,600  sq  yd 
=  27,878,400  sq  ft 
=  2.59  sq  kilo 

i  acre  =  i6o  sq  rd 
=  4840  sq  yd 
=  43560  sq  ft 
=  4046 . 87  sq  meters 
=  0.0404687  hectare 


REFERENCE   TABLES 


259 


I  square  yd  =0.83613  sq  meter 
I  square  foot  =  o.iin  sq  yd 
=  144  sq  in 
=  o .  0929  sq  meter 
=  929.03  sq  cm 

I  square  inch  =  o .  006944  sq  ft 
=  6.4516  sq  cm 

i  square  mil  =  0.00001  sq  in 

=  0.00064516  sq  mm 

I  square  kilometer 

=  0.3861  sq  mi 
=  247. 105  acres 
=  1,000,000  sq  meters 

I  square  meter  =  1 . 196  sq  yd 
=  10.7639  sq  ft 
=  1550  sq  in 

I  square  centimeter 

=  0.155  sq  in 

=  0.0001  sq  meter 

I  square  millimeter  =  1 550  sq  mils 
=  0.00155  sq  in 

i  hectare  =  2 . 47 1  acres 
=  107,639  sq  ft 

CUBIC  EQUIVALENTS 
I   cubic  yard  =27  cu   ft  =46,656  cu 
in  =  o .  76456  cu  meter 

I  cubic  foot  =0.037  cu  yd 
=  1728  cu  in 
=  7 . 4805  U  S  gals 
=  6.2321  Imp  gal 
=  0.0283  cu  meter 
=  28317  cu  crn 

I  cubic  inch  =  16.  3872  cu  cm 

I  cubic  meter  =  i .  30794  cu  yd 
=  35. 3145  cu  ft 
=  61,023.4  cu  in 


i  cubic  decimeter  (i  liter) 

=  0.03531  cu  ft 
=  61.0234  cu  in 
=  1000  cu  cm 
=  0.26417  U  S  gal 
=  0.22  Imp  gal 


CAPACITY 

I  bushel  U  S  =  i .  24445  cu  ft 
=  2150.4  cu  in 

I  bushel  Br  =  i .  28368 

i  gallon  U  S  =  o.  133681  cu  ft 

=  231  cu  in 

=  3-78543  liters 

=  3785  cu  cm 

=  4qts 

=  8  pts 

A  cylinder  7  in  diameter  and  6  in 
high  has  a  capacity  of  230.9  cu  in — 
practically  i  gal. 

i  gallon,  Imperial  =  277.274  cu  in 

=  4.5437  liters 
i  quart  =  57. 75  cu  in 

=  946.25  cu  cm 

i  fluid  ounce  =  i .  8047  cu  in 
=  29.574  cu  cm 


WEIGHT 

I  ton,  long  =  2240  Ibs  (av) 

=  1.016  met  tons 
=  1016.05  kg 

I  ton,  short  =  2000  Ibs 

=  0.907  met  ton 
=  907  kilo 

I  pound  avoirdupois  =  16  oz 

=  7000  grains 
=  0.4536  kg 
=  453 -59  gram 


260 


APPENDIX 


i  pound  troy  =  12  oz 

=  576o  grains 
=  372.631  grams 
kg 


I  ounce  avoirdupois  =  437  .  5  grains 
=  0.0625  Ib 
=  28.35  grams 

i  ounce  troy  =  480  grains 
=  0.0833  Ib 
=  31.  i  grams 

I  metric  ton  =0.9842  long  tons 
=  1  .  1023  short  tons 
=  2204.62  Ibs 
=  1000  kg 

I  kilogram  =  2  .  20462  Ib  av 
=  2  .  6792  Ib  troy 
=  35-274ozav 
=  33.140  oz  troy 

I  gram  =  15.  432  grains 

=  1000  mg 

=  I  cu  cm  watef 
i  grain  =  o  .  0648  gram 


WORK  EQUIVALENTS 

I  horse  power 

=  33,000  ft  Ib  per  min 
=  i  ,980,000  ft  Ib  per  sec 
=  273,746  kg  meters 
=  2,685,443  joules 
=  2544. 99  Ib 
=  641.326  kilo 

i  Br. therm. unit  =  i  Ib  i°  F 
=  778  ft  Ib 
=  0.252  kgC° 
=  107 . 56  kg  meters 

I  joule  =  0.7373  ft  Ib 

=  o.  1 02  kg  meter 


I  kilogrammeter 

=  7.89  h  p  min 
=  o.  1315  h  p  sec 
=  7.233  ftlb 
=  0.8  met  h  p  min 
=  0.0133  met  h  p  sec 
=  9.81  joules 
i  kilogram  degree  C° 

=  3087.35  ft  Ib 
=  3.968  deglb  F° 
=  426 . 84  kg  meters 

i  metric  horse  power 

=  1,952,910  ft  Ib  (hour) 
=  2,648,700  joules 

PRECIPITATION 

I  acre  foot  =  43, 560  cu  ft 

=  325,850. 58  US  gal 
=  o .  6234  U  S  gal  per  sq  ft 
=  1233 . 46  cu  meters 
=  1344.2  tons 

I  acre  inch  =3630  cu  ft 

=  27154.21  U  S  gal 
=  102.8  cu  meters 
=  112.01  tons 

I  acre  centimeter 

=  0.24542  U  S  gal  per  sq  ft 

ATMOSPHERIC  EQUIVALENTS 
t=32°     P  =  29. 92  in  or  760  mm 

I  cubic  yard  of  air  =  2. 17888  Ib  av 
=  988.615  grams 

I  cubic  foot  of  air  =  0.807  Ib  av 

=  565.061  grains 
=  36.615  grams 
=  i  .291  oz  av 

I  atmosphere  =  29. 92 1  in  of  mercury 
=  760  mm  of  mercury 
=  33 . 9  ft  of  water 
=  14.7  Ib  per  sq  in 
=  i .  0333  kg  per  sq  cm 


REFERENCE   TABLES 


261 


I  metric  atmosphere 

=  0.9678  atmosphere 
i  cubic  meter  of  air  (32°  F) 

=  2.85lb 

= i . 293  kg 


i  inch 


VELOCITY 

per  second  =  300  ft  per  hour 

=  1 52 . 4  cm  per  min 

i  foot  per  second 

=  60  ft  per  min 
=  3600  ft  per  hour 
=  0.682  mi  per  hour 
=  i .  097  kilo  per  hour 

I  mile  per  hour 

=  i .  466  ft  per  sec 

=  88  ft  per  min 

=  0.447  meter  per  sec 

=  26.822  meters  per  min 

I  mile  per  min 

=  5280  ft  per  min 

=  96.5608  kilo  per  hour 

=  16.035  kilo  per  min 


i  kilometer  per  min 

=  2.2369  mi  per  hour 
=  96.5608  ft  per  min 

MISCELLANEOUS 

i  knot 

=  rate  of  i  nautical  mile  per  hour 

i'  of  longitude  at  equator 

=  I  855 -345  meters 
=  6087.15  ft 

i'  of  latitude  at  equator 

=  1842.787  meters 
=  6045. 95  ft 
=  i.  14507  st  mi 

i'  longitude  at  equator 

=  i .  15287  st  mi 

Equatorial  radius  of  earth 

=  3962 -57 1  mi 

Polar  radius  of  earth 

=  3949-67  mi 

I  cu  ft  of  ice  at  32°  =  57 . 5  Ibs 
i  Ib  of  ice  at  32°  =  o.oi74  cu  ft 
i  inch  of  mercury  in  the  barometer 
tube  =  13 . 5956  in  of  water 


262 


APPENDIX 


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264 


APPENDIX 


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Tf  »OvO   t^OO    ON  O   HH    (v)    to 
CM    OOOO    ThCM    HH    ONt^.10 

vO    ON  HH    rt-  t^  O    to  »OOO    HH 
CM    IO  ON  CM    IO  ON  (M    lOOO    CM 

HH      HH      HH      CM      CM      CM      tO 

l^ 

^O  t^OO    ON  O   HH    N    to  rj-  iO 
ONr^»OtOCM    OOOvO    Tj-  CM 

CM    lOOO    HH    r^-  t^  ON  CM    lOOO 
CM    lOOO    CM    lOOO    HH    lOOO    HH 

HH     HH     HH     CM     CM     CM     tO 

vO 

IO  r|-  fO 

oo  a  o  HH  CM  to  TJ-  io>o  ^ 

MD      TftOHH      ONt^lOtOHH      ON 

ON  CM   >OQO    O   fOO   ON  CM   •**• 

HH      lOOO      HH      lOOO      HH      ThOO      HH 
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Tt-  tO  CM 
O    HH    CM    to  Tj-  IOVO   t^OO    ON 

"tf-CM  Ooovo  ^I-CM  ooovo 

iO 

^OCMCM-^-l^OtOvOOOHH 

HH      Tt-OO      HH      TtOO      HH      Th   t^    HH 
HH     HH     HH     CM     CM     CM      tO 

Tj- 

tO  CM      HH 

CM   tOrt-iO^O  t^oo   ON  O   O 

HH      ON   t>.   »O    tO   HH      ON   t^^O      ^" 

to  »OOO    HH    Tl-t^ONCM    >OOO 

HH      TJ-t^HH      Tj-t^O      n-I^O 

HH    HH    HH    CM    CM    CM    tO 

tO 

tO    HH      ON 

T!-  iO  iO  t^OO  00   ON  O   HH   CM 
OCv£>    rfCM    OOOvS    lOfOHH 

ON  CM    lOOO    HH    COO    ON  CM    »O 
^•t~>>O   ^t^O   tOf^-O 

HH     HH      HH      CM      CM      CM      fO 

M 

CM    O  00 
O    t^t^ONONO    HH    CM    tOTh 

«O  to  HH   ON  l^vo   rh  CM   O  oo 

O  ON  CM   Tt-  1^  O   tOO   ON  HH 
tO  t^  O   to  1^  O   tOO   O 

HH     HH      HH      CM      CM      CM      tO 

N 

HH      ON  t^ 

OOOOONHHHHCMfO-^-  lOO 
CM    OOOt^lOtOHH    ONt^lO 

fOO  00    HH    -HT  l^  O    CM    lOOO 

too  o  too  o  too  ON 

HH     HH     HH     CM     CM     CM     CM 

00   t>»  IO 

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OOO   rfCM   OOOO   rJ-CM    C 

» 

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CM    lOOO    HH    T+-O    ON  CM    »OO< 
tOO   ON  tOO   ON  CM  O    ON  C 

HH      HH      HH     CM      CM     CM      t 

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0 

028&S-8>SR<g8J 

i 

4 

266 


APPENDIX 


* 

QN  cO  t^»  M   10  ON  ^O  t^«  ^<  *O 

« 

g  N  rj-^ONfN  2-r-O  PJ  jo 

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CO        c  ^ 

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rOt^~   lOONcOl^.'-'  to  ON 

c8    a8 

00 

—    O   *O  *•*  \O    >"*   t^*  CN  00    cOOO 

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rn  r^»  rCOO   ^h  ON  ^t"  O   *O  >^  vC 

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p  £T<»  Qao  $  o^  Io  <?  °^  S? 

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REFERENCE   TABLES 


267 


ON 

tOO  r^-Tt-MOO   »OM   ONVO 

^•oo  1-1  10  ON  (s  vo  o  to  r>» 

_,   iO  rf-  rf-  tO  <N   M    **   "-i   O   ON 

.5  to  r^  -<  »o  ON  to  t^  HH  >ooo 

£  °        ^ 

ON 

P--S8 

00 

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lOOO   M  vO   ON  to  t-^  O   TTOO 
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g   0    —  «   0 

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vO   ON  tO  t^«  >->   ^t"OO   CS   1O  ON 
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ON 

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i-i   (S   to  "*•  >OvO   l^oo   ON 

KM 

Bc 

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do 

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Jd 

268 


APPENDIX 


HHOOlOMOOlOt-lOOTt-hHOOTt-l-ir^Tj-l-lt^^l-lt^. 

ON  OO  OO  00  l^  t1*^  r^  \Q  vO  ^O  *O  iO  10  ^"  Tj"  *^  to  ^O  tO  CS 

22222222222222222222 

O     ONOO     i^«.vO     to    ^-    CO    CN     >-*     O     ONOO     1^-vO     to^cOCN     *-> 
WCNCNCNCNCNCNCNCNCN'CNCNCNCNCNCNCNCNCNCN 

ONVO     <N     ONQ     CNI     ONtOCN     ON   to    N     ONtOHOO     tOCNOO     to 
tOiOtOTj-^-Tj-rOcOcOCN     CN     CN     1-1     HH     I-H     Q     O     O     ONON 

oooooooooooooooooooo 

8ONOO     t^vO     iOTj-cO<Ni     I-H     O     ONOO     t^vO     tOrf-cOCN     •-" 
ONONONONONONONONONONOOOOOOOOOOOOOOOOOO 

COCNCNCNCNCNCNCNCNCNCNCNCSCNCNCNCNCNMCN 

t^rt-Q     t>.cOO    t^cOOvO     cOOvO     cOOvO     cOONVO     CO 

CN     CN     CN     1-1     t-i     1-1     O     O     O     ONONONOOOOOO     J^^vOvOvO 
CNCNCNCNCNCNCNC^CNi-i>-ii-ii-(i-H>-i(-iHH>HhHhH 

OOOOOOOOOOOOOOOOOOOO 
O     ONOO     t~»vO     tOTj-rOCN     HH     O     ONOO     t^  vO     to    ^-    CO    CN     ** 

-,         —         ._         —         __         |^'      HHHHhHHHQQQOOQQOO 

OOOOOOOOOOOOOOOOOOOO 
CO    co    cO    CO    co    CO    co    co    CO    CO    co    co    CO    co    CO    co    co    CO    co    co 

lOMOO      Tt-HHOO      rl-MOO      •*    1-1      I^Ti-i-i      t^.-^-O     t^rj-O 

ONONOOOOOO    t^.t>-t^.vOvOvO     toiOtO'^-Tt'^cOcOcO 
(NlCNCNMCNCSCSMCNMCNl(N)(NCNC^CNCNCNCNiM 

OOOOOOOOOOOOOOOOOOOO 

Tt-tocOcOcOcOcOcOcOcOcOCN     CN     CN     CN     CNI     CN     CN     CN     CN 

d   d   d   d   d   o   d   d   d   o   d   d   d   d   o   o   d   d   d   d 

cOcOcOcOcOcOcOcocOcOcOcOcOcOcOcOcOcocOcO 

CN     ONVO     CN     ONtOCN     ONtOCN     ONtOcNOO     tOCNOO     to    HH    oo 

vO     to    to- to    Tt'Tt''^cOcOcOCNCNHi^HHi-iOOOON 
cOcOcOcOcOcOcOcOcOcOcOcOcOcOcOcOcOcOcOCN 

O     ONOO     t^vO     to^cOM     M     O     ONOO    b«.vO     tO^t-cOCN     >-> 
vO     lOiOlOtotOlOlOlOlOlO'^-'rj-Tj-Ti-rJ-Tt-Tj-Ti-rl- 

d   d   d   d   d   d   d   d   d   d   d   d   d   d   d   d   d   d   d   d 

cocococococococococococococococococococo 

O  f^*  co  O  1^*  cO  O  vO  cO  O  vO  co  ON  vO  cO  ON  vO  co  ON  vo 

cOCN)CNCN)t-i>-ii-iOOOONONOOOOOOl^J^l^vOvO 

o"ooooooooooooooo'oooo 

O     ONOO     t~*vO     tOrt-cOCN     1-1     O     ONOO     1^-vO     tO-^cOCN     >-< 

6  6  6  6  6  6  6  6  6  6  6  6  6  6  6  6  6  6  6  6 

cocococococococococococococococococococo 

oo    IOMOO    Th  »-i   oo    ^M    t^   T|-  M    t^.T)-O    t^-rj-o    l^-co 
ONONONONOOOO     t^.t^.t^>.vOvOvO     totOtOrl-TJ-r^-rOcO 


t^vO     tO^cOCN     I-H     O     ONOO     t^.vO     tOT^-rOCN     w 
ONONONONONONONONOOOOOOOOOOOOOOOOOO 


0000 
CO    co    CO    CO 


ooooooooooo 

cococococococococococo 


REFERENCE   TABLES 


269 


TABLE  12  —  BAROMETER  INCHES  TO  KILOBARS  —  Continued 

IO 

CM    OO 

10    CM 

00     10 

M     00 

10     M 

00      Th 

M     00 

rf    M    00 

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cs 

$ 

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vO    NO 
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t->.    t^ 

vO     IO 

ON    ON 

NO     NO 

ON    ON 

MD     IO 

NO     NO 

ON    ON 

to  10 

VO     NO 
ON    ON 

V^NO^ 
ON    ON 

"3-    fC    rO    CO   <N< 

NO     NO     NO     NO     NO 

8 

00 
CM 

ON  00 

to  to 

00    00 

CM    CM 

IO    IO 

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10    10 

00    00 
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8$ 

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tO 

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NO     (M 

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0)     ON 

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ON    10 

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ON    ON 

ON    ON 

ON    ON 

ON    ON 

ON    ON 

ON    ON 

ON    ON 

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£ 

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t>-    rf 

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00    00    00    00 
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to    to 
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ON    ON 
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0>        W 

^0°, 

c?c? 

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10      Tj-      tO 

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n 

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to 

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CM        HH 

8  8 

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8  § 

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ON    ON  00    00 
ON    ON    ON    ON 
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00     l^. 

%% 

l^  vO    NO 
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10    10 

ON    ON 
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ON    ON 
N     N 

rf5    M 
IO    IO 

ON    ON 
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to  to 

ON    ON 
CM     CM 

$% 

c?r? 

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$33 

CM 
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ON    ON    ON 
CM     CM     d 

81 

$ 

270 


APPENDIX 


ONIOCM     ON    IO    CM    00     tOCMOO     »Oi-iOO     lOi-iOO     iO    i-i    00     rf- 

t^.t^t^vOvOvO     lOiOiO-rf-Tj-rJ-cOCOcOCM     CM     CM     >-i     >-> 
04O4O404CMO4CMCM04O4O40404O4O4CM04O404CM 

O     ONOO     t^  VO     10    Tl-    CO    CM     i-i     O     ONOO     r>-  VO     IO    T*-    CO    04     ^ 
Tj-cOcOcOcOcOcOcOcOcOcOO4     CM     04     04     04     04     CM     CM     04 

0404C4CMCMO4CMCM04CM04040404040404040404 

vO     cOOvO     cOOvO     cOONVO     CO   ON  vO     CM     ON  vO     CM     ONIO04 
T}-    rt-    ^    CO    CO    cO    O4     CM     HM     i-i     M     O     O     O     ONONONOOOOOO 

O     ONOO     t>»  VO     »O    Tj-    CO    04     HH     O     ONOO     t^vO     lOrf-cOCM     M 
\O     lOlOlOlOlOlOiOlOlOlO^'^^'T^Tj-TJ-Tf^'Tj- 

040404CM04040404O4CMO4CM04O4040404CMCM04 

HH     1-1     O     O     O     ONONONOOOOOO     t^t>.t^vOvOvO     lOiOiO 
^^   Tf*   ^h    ^h   ^"    cO    co    co    co    co    co    co   co    co   co    co    co    co    co    CO 

O     ONOO     t>»vO     lO^t-fOCM     >-i     O     ONOO    (>«vO    lO^J-fOCN     i-i 
00    t^.l>«t>.t^t^.t^t^t^t^t^vOvOvOvOvOvOvOvOvO 

CM04040404CMCMCMCM0404040404CMCMCMCM0404 
CMOO     IO0400     lOi-iOO     IO    i-i    00     Ti-MOO     Tl-i-iOO     Tj->-i     IN. 

8ONOO     t^vO     lOTj-rOCM     i-i     O     ONOO     t^vO     lOrj-cOCM     •-" 
ONONONONONONONONONONOOOOOOOOOOOOOOOOOO 

oo    t^r^t^t^t^t^i^i^t^r^.t^t^r^t->.t^.t>.t^t^^ 

CMCMO4040104O40ICMCMCMCM0404040404CM04O4 

OvO    COONVO    cOONVO    cOONVO    M    ONVO    04     ONiO04    ONIO 

lO^Tj-cOcOcO04     04     04     M     i-i     i-i     O     O     O     ONONONOOOO 
lOiOiOiOiOiOiOiOiOiOiOiOiOiOiOrt'ThTj-'^-rt- 

ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON  ON 

O  ON  OO  t^»  vO  IO  ^h  CO  04  M  O  ON  OO  t***  VO  IO  Tt"  CO  04  HH 

fS|HHMHHl— I^HhHhHHHHHH-OOOOOOOOO 

OOOOOOOOOOOOOOCOOOOOOOOOOOOOOOOOOOOOOOOO 
04CMO4O404O40404CM04O4O4040404040404CM04 


H-I    t^    CO    O 


cO    O    vO     co 


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REFERENCE   TABLES 


271 


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272 


APPENDIX 


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REFERENCE  TABLES 


273 


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274 


APPENDIX 


O   N    ^NO  00    O   N    ^t*NO  00    O    N    ^-NO  00    O   N    rJ-rhO)    O  00  NO    ^h  N 
(S    OOONO    ^rO1^    ONt>»|O'^M    OOOO    lOrO*^    O    W    ^»Ol>»ON'-' 

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CJ  WNWWNCjNH^HHhHHHMHHHHI-,^  _J_ 

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U 


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vOvO 


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REFERENCE  TABLES 


275 


OoosO    Tf-pj    OoosO    TJ-PI    OoosO    rj-pi    OoosO    rhpi    OoosO    rhpj 
tO^J-sOoo    Q    PI    fO  to  t~*  ON  HI    M    T)-  SO  00    O    HI    f<5  to  I**  ON  O    PI    rj-so" 

fc  1-HHII-lt-lMCIPIPIPIPIPIPIPIPIPIPIPIPIPIPIPICIPIPIPI 

I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I  I   I   I   I  I 

to  sO    t>«  OO    ONO    HI    M    rO  ^  to  sO    r^  00    ONO    HI    ci    r<}  Tf  to  sO    t^QO    ON 
PI    PI    PI    PI    PI    tOfOrOfOrOfOtOcOfOfi^'^'TJ-Tt-TJ-Tj-^-Tt-^-Tt' 

u  TTTTTTTTTTTTTTTTTTTTTTTTT 

& 

OOOsO    ^  PI    OoosO    rhPi    OoosO    TJ-PI    OoosO    O    O    OoO^O    ^Pl 
00    ONHI    coiOl^.00    O    PI    rJ-sO    I^ONHI    retOsOOO    rfPi    rf-iOt^ONHi 

TTTTTTTTTTTTTTTTTTTTTTTTT 
i  i  i  i  i  i  i  i  i  i  i 

P|HiOONOOt>.sOlOTt"f<5PIHiOON 
sO  sO   sO    *O  lO  *O  ^O  *O  lO  *O  *O  to  to  Tt" 

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Pi    OoosO    TJ-PI    OoosO    ^Pi    OoosO    ^Pl    OoosO    •<*• 

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u      i    i    i    i    i    I   l    l   l    l    I    l   l   I    I    l    I    I    I    l    l   l   I   I   I 

00    h»\O    tOrJ-rOPI    HI    O    ONOO    l^«sO    to^i-rOPI    HI    O    ONOO    t>-sO 
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PIPIPIPIPIPIPIPIPIPIPIPIPIPIPIPIPIMPIPIPIPIPIPIM 


276 


APPENDIX 


0  00  VO  <*  M    O  00  vo    rf-  04    O  00 
00    ON  HH  ro  IO  t^OO    O    04    -d-vO  t~> 

3.3.«  3-3-3-3-33 

1  I       I  I       I       I       I       I       I       I       I  I       I       I 


CO  CO 


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I   I   I   I   I   I   I   I   I 


u 


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04O4O4O404040404O40404O40404O404O4040404040404O4 
I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  1  I  I  I  I  I 


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,0  04     04     04     04     ~ 

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lOvO    l^  00    ON  O    >->    O4    cO-^-iOvO    t^OO    OvO    "-1    04    co^-iOvO    t^oo    O\ 
04     04    04    04    04     COCOCOCOcOcOCOcOCOcO^-'<i-Tt-Tj-TJ-TJ-Tt-TJ-Tj-Th 
T  \  O40404040404040404040404O404040404O4O40404O404O4O4 

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I   I   I   I   I   I   I   1   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I   I 

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I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I        I 

Q  HH  d  tOrt"iO\O  t^OO  O^O  »-<  d  tO^^O^Ot^OO  O^  O  ^  M  tO^h 
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REFERENCE  TABLES 


277 


TABLE  16 — FAHRENHEIT  DEGREES  TO  CENTIGRADE  DEGREES 


F° 

C9 

F° 

C° 

F° 

c° 

F° 

C° 

F° 

C° 

-40 

—40.0 

12 

—  ii  .1 

63 

17.2 

114 

45-6 

165 

73-9 

-39 

-39-4 

13 

—  10.6 

64 

17.8 

H5 

46.1 

1  66 

74-4 

-38 

-38.9 

14 

—  10.  0 

65 

18.3 

116 

46.7 

167 

75-0 

-37 

-38.3 

15 

-  9-4 

66 

18.9 

117 

47-2 

1  68 

75-6 

-36 

-37-8 

16 

-  8.9 

67 

19.4 

118 

47-8 

169 

76.1 

-35 

-37-2 

17 

-  8.3 

68 

20.  o 

119 

48.3 

170 

76.7 

-34 

-36-7 

18 

—  7-8 

69 

20.  6 

120 

48.9 

171 

77-2 

-33 

-36.1 

19 

-  7-2 

70 

21.  I 

121 

49-4 

172 

77-8 

-32 

-35-6 

20 

-  6.7 

71 

21.7 

122 

50.0 

173 

78-3 

-3i 

-35-0 

21 

-  6.1 

72 

22.2 

I23 

50.6 

174 

78.9 

-30 

-34-4 

22 

-  5-6 

73 

22.8 

I24 

5i.i 

175 

79-4 

-29 

-33-9 

23 

-5-0 

74 

23-3 

125 

51-4 

176 

80.0 

-28 

-33-3 

24 

-  4-4 

75 

23-9 

126 

52.2 

177 

80.6 

-27 

-32.8 

25 

-  3-9 

76 

24.4 

127 

52.8 

178 

81.1 

-26 

-32.2 

26 

-  3-3 

77 

25.0 

128 

53-3 

.  179 

8i-7 

-25 

-31-7 

27 

-  2.8 

78 

25-6 

I29 

53-9 

1  80 

82.2 

-24 

-3i.i 

28 

—  2.2 

79 

26.1 

130 

54-4 

181 

82.8 

-23 

-30.6 

29 

-  i.7 

80 

26.7 

131 

55-0 

182 

83.3 

—  22 

-30.0 

30 

—  I.I 

81 

27.2 

132 

55-6 

183 

83-9 

—  20 

-29-4 

31 

-  0.6 

82 

27.8 

133 

56.1 

184 

84.4 

-19 

-28.3 

32 

0.0 

83 

28.3 

134 

56.4 

185 

85-0 

-18 

-27.8 

33 

0.6 

84 

28.9 

135 

57-2 

1  86 

85-6 

-17 

-27.2 

34 

i  .1 

85 

29.4 

136 

57-8 

187 

86.1 

-16 

-26.7 

35 

i-7 

86 

30.0 

137 

58-3 

1  88 

86.7 

-15 

-26.1 

36 

2.2 

87 

30.6 

138 

58.9 

189 

87-2 

-14 

-25-6 

37 

2.8 

88 

3I-I 

139 

59-4 

190 

87.8 

-13 

-25.0 

38 

3-3 

89 

31-7 

140 

60.0 

191 

88.3 

—  12 

-24-4 

39 

3-9 

90 

32.2 

I4I 

60.6 

192 

88.9 

—  II 

-23-9 

40 

4-4 

91 

32-8 

142 

61.1 

193 

89-4 

—  10 

-23-3 

4i 

5-0 

92 

33-3 

143 

6i.7 

194 

90.0 

-  9 

-22.8 

42 

5-6 

93 

33-9 

144 

62.2 

195 

90.6 

-  8 

—  22.2 

43 

6.1 

94 

34-4 

M5 

62.8 

196 

91.1 

-  7 

-21.7 

44 

6-7 

95 

35-0 

146 

63.3 

197 

9L7 

-  6 

-21.6 

45 

7-2 

96 

35-6 

147 

63-9 

198 

92.2 

—  5 

—  20.6 

46 

7-8 

97 

36.i 

148 

64.4 

199 

92.8 

-  4 

—  20.0 

47 

8-3 

98 

36.7 

149 

65.0 

200 

93-3 

-  3 

-19.4 

48 

8-9 

99 

37-2 

150 

65-6 

201 

93-9 

—  2 

-18.9 

49 

9-4 

100 

37-8 

151 

66.1 

202 

94-4 

—  I 

-18-3 

50 

10.  0 

101 

38.3 

IS2 

66.7 

203 

95-0 

0 

-17.8 

5i 

10.6 

102 

38.9 

153 

67.2 

204 

95-6 

I 

-17*2 

52 

ii.  i 

103 

39-4 

154 

67.8 

205 

96.1 

2 

-16.7 

53 

ii.  7 

104 

40.0 

155 

68.3 

206 

96.7 

3 

-16.1 

54 

12.2 

105 

40.6 

156 

68.9 

207 

97-2 

4 

-15-6 

55 

12.8 

1  06 

41.1 

157 

69-4 

208 

97.8 

5 

-15-0 

56 

13  3 

107 

41-7 

158 

70.0 

209 

98.3 

6 

-14.4 

57 

13.9 

1  08 

42.2 

159 

70.6 

210 

98.9 

7 

-13  9 

58 

14.4 

109 

42.8 

1  60 

7I-I 

211 

99-4 

8 

-I3-3 

59 

15.0 

no 

43-3 

161 

71-7 

212 

100.  0 

9 

-12.8 

60 

15.6 

III 

43-9 

162 

72.2 

10 

—  12.2 

61 

16.1 

112 

44-4 

163 

72.8 

ii 

-ii  7 

62 

16.7 

H3 

45-0 

164 

73-3 

( 

i 

278  APPENDIX 

TABLE   17 — BOILING  POINT  OF  WATER  (F)  AS  AFFECTED  BY  PRESSURE 


p 

inches 

t° 

P 

inches 

t° 

P 

inches 

t° 

P 

inches 

t° 

16.79 

184.0 

19.96 

192.0 

23-59 

200.0 

27-73 

208.0 

16.97 

184-5 

20.18 

192.5 

23-84 

200.5 

28.00 

208.5 

17.16 

185.0 

20.39 

193.0 

24.08 

201.0 

28.69 

209.0 

17-35 

I85-5 

20.61 

193-5 

24-33 

201.5 

28.56 

209.5 

17-54 

186.0 

20.82 

194.0 

24.58 

202.0 

28.85 

210.0 

17-74 

186.5 

21.05 

194-5 

24-83 

202.5 

29-15 

210.5 

17-93 

187.0 

20  .  26 

195  -o 

25.08 

203.0 

29.42 

211  .O 

18.12 

187.5 

20.49 

195-5 

25-33 

203.5 

29.71 

2II.5 

18.32 

188.0 

21.71 

196.0 

25-59 

204.0 

30.00 

212.0 

18.52 

188.5 

21-95 

196.5 

25-86 

204.5 

30.30 

212.5 

18.72 

189.0 

22.17 

197.0 

26.11 

205.0 

30.59 

213.0 

18.92 

189.5 

22.41 

197-5 

26.38 

205.5 

30.89 

213-5 

19.13 

190.0 

22.64 

198.0 

26.64 

2O6.O 

31.10 

214.0 

19-33 

190-5 

22.89 

198-5 

26.91 

206.5 

19-54 

191.0 

23-11 

199.0 

27.18 

207  .  o 

19  74 

191-5 

23-36 

199-5 

27-45 

207-5 

TABLE  18 — BOILING  POINT  OF  WATER  (C)  AS  AFFECTED  BY  PRESSURE 


P 

mm 

t° 

P 

mm 

t° 

P 
mm 

t° 

P 

mm 

t° 

680 

96.92 

710 

98.11 

740 

99.26 

770 

100.37 

682 

97.00 

712 

98.18 

742 

99-33 

772 

100.44 

684 

97.08 

7H 

98.26 

744 

99.41 

774 

100.51 

685 

97.12 

715 

98.30 

745 

99-44 

775 

100.55 

686 

97.16 

716 

98-34 

746 

99.48 

776 

100.58 

688 

97.24 

718 

98.42 

748 

99  56 

778 

100.66 

690 

97-32 

720 

98.49 

750 

99-63 

780 

100.73 

692 

97-40 

722 

98.57 

752 

99.70 

782 

100.80 

694 

97.48 

724 

98.65 

754 

99.78 

784 

100.87 

695 

97-52 

725 

98.69 

755 

99-82 

785 

100.91 

696 

97.56 

726 

98.72 

756 

99.85 

786 

100.94 

698 

97-63 

728 

98.80 

758 

99-93 

788 

101  .02 

700 

97.71 

730 

98.88 

760 

IOO.OO 

790 

101.09 

702 

97-79 

732 

98.95 

762 

100.04 

792 

101.16 

704 

97.87 

734 

99  03 

764 

100.  II 

794 

101.23 

705 

97.91 

735 

99-07 

765 

100.  18 

795 

101  .26 

706 

97-95 

736 

99.10 

766 

100.22 

796 

101.30 

708 

98.03 

738 

99.18 

768 

100.29 

798 

101.37 

REFERENCE   TABLES 


279 


TABLE  19 — QUANTITY  OF  RAINFALL  IN  Cu  FT  AND  U  S  GALLONS  PER  ACRE 


Inches  of 
rainfall 

Cuft 
per  acre 

Gallons 
per  acre 

Inches  of 
rainfall 

Cuft 
per  acre 

Gallons 
per  acre 

O.OI 

36.3 

271-5 

0.  10 

363 

2715-4 

O.O2 

72.6 

543 

0.20 

726 

5430 

0.03 

108.9 

8i5 

0.30 

1089 

8146 

0.04 

H5-2 

1086 

0.40 

1452 

10862 

0.05 

181.5 

1358 

0.50 

1815 

13577 

0.06 

217.8 

1629 

0.60 

2718 

16293 

0.07 

254-1 

1900 

0.70 

2541 

19007 

0.08 

290.4 

2171 

0.80 

2904 

21722 

0.09 

326.7 

2442 

0.90 

3267 

24438 

o.  10 

363-0 

2715 

i!oo 

3630 

27153 

The  U  S  or  Queen  Anne  gallon  used  in  the  foregoing  table  contains  231 
cu  in  or  0.1368  cu  ft.  The  Imperial,  or  British  gallon  contains  277.3  cu  m- 
To  reduce  U  S  to  Imperial  gallons  multiply  by  0.83^. 

To  find  the  quantity  of  rainfall  per  square  mile,  multiply  the  quantity 
per  acre  by  640. 

One  inch  of  rain  per  acre  is  at  the  rate  of  113  tons  per  acre  or  7320  tons 
per  sq  mi. 


TABLE  20 — DEPTH  OF  WATER  IN  A  STANDARD  S-INCH  GAUGE 
CORRESPONDING  TO  THE  WEIGHT  OF  SNOW  OR  OF  RAIN. 


Weight 
Pounds 

.00 

.01 

.02 

•03 

.04 

•05 

.06 

.07 

.08 

.09 

in 

in 

in 

in 

in 

in 

in 

in 

in 

in 

0.0 

.00 

.01 

.01 

.02 

.02 

•03 

•03 

.04 

.04 

•05 

0.  I 

.06 

.06 

.07 

.07 

.08 

.08 

.09 

.09 

.  10 

.  10 

0.2 

.  II 

.12 

.  12 

•13 

•13 

•14 

•14 

•15 

•15 

.16 

0-3 

•17 

•17 

.18 

.18 

•19 

•19 

.20 

.20 

.21 

.22 

0.4 

.22 

•23 

•23 

.24 

.24 

•25 

•25 

.26 

.26 

.27 

0-5 

.28 

.28 

•29 

•29 

•30 

•30 

•31 

•31 

•32 

•32 

0.6 

•33 

•34 

•34 

•35 

•35 

•36 

•36 

•37 

•38 

•38 

0.7 

•39 

•39 

.40 

.40 

•4i 

•41 

•42 

•43 

•43 

•44 

0.8 

•44 

•45 

•45 

.46 

.46 

•47 

•47 

.48 

•49 

49 

0.9 

•50 

•50 

•5i 

•51 

•52 

•52 

•53 

•54 

•54 

•55 

One  pound  equals  0.5507  in. 


280 


APPENDIX 


>O  N  O  ro  t^vO  oo  to  ro  t^vO  n  >-• 
O  1-1  fOvO  O  vO  fO  W  (N  rO^O  i-i  (N 
rOOO  fOOO  *^-  o\  10  i-t  t>.  ro  O^^O  t~» 


l^  rOOO   O   O 

O  vO 

OO  10  rO  •"• 


l^»  t>.OO   ON  O   O   i-*   M    fO 
>-i>-iCiM<N|CS(SCS>-<NrNC*<NiNC4MfOtOtOtOfO 


8^2  S 


O   tOvO  00   1-1 
oo  oo  oo  oo 


^O'-'vOt-i   -^"00  (SvOOiO'-OOOOOO 

»oo  »ors 

Tj-r^i-i 


OiO'-O 

a\>-H  too 

O  *O  Q\  n- 


n  vO  O  *O  Q\  n-00 


O   O   "-"   >->   **   CS 


* 


o  HH 


o  I-"  n  <o  •<*•  io\o  i^oo  a\  o  « 

oo  oo  oo  oc  oo  oo  oo  oo  oo  oo  o\  o^ 


vO  rC  rooo  »OOO  if)  t^.  *$•  \r>  *-<  rsoo  O  t^O  ON^tOO  M  M  I^O  CN 
f)vO  O\MVOOC4O^OM  O\vO  cOMsOOOO'-irO  ^OO  •-•  \o  O 
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a 

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t^ONi-i    tOiOt^O   (N)   lOOO   i-i    -rj-l^  O    -^-00   «-i   iO  O    rl-00    fOOO   fOOO 
to  to  ^"  ^i~  *^  ^  ^O  ^O  O   lO^O  \O  ^O  t^*  l^»  r^OO  00   ON  ON  ON  O   O   HH   »-H 

06600066006006066666  6  ~  >-<'  «  w 

^  lO-j-fOCM'-'O'-infO'^'  tOO   t>-OO   ON  O    i-i   (N)    fO  ^i"  lOvO   t">-OO   ON 

IIIII-H+  MMMMMMMM«« 

iO  O  ^O   CS    ONVO    ^i"  *•<   O  OO   r^*^O   t>*  t>>»OO   O   CNI    ^00   CS  \O   CN|  OO   iO  to 

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o  o  o"  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o 

^  O   ONOO   t>.O   iO  ^- fO  CJ   <->   O   ONOO   l~-.vO   iO  rt- to  (N)    1-1    O   ONOO  t>.\O 

tO(N|fNltNrsM(S(N)O)OJMt-ii-ii-Hi-(i-ii-ii-ii-(i-i(-i 

I  I  I  I  I  I  I  I 


REFERENCE  TABLES 


281 


O  *O  O  O  O  1C  O  >O  O  lOOiOOiOOiOQiOOiO 
O  CO  t^  hH  OO  lOcOOCO  iO  CO  O  00  uocOOOO  lOcOiO 

10  10  10  NO   r-»  ON  I-H    co  "3-  NO  oo    o    hH    coiot^.oo    o    CN    CN 

cOior-ONONONOOOOOhHhHhHhHhHhHc^oirh 
OOOOhHCvjThiONOr^-OOONOhHC^cOrh'ONONO 
(N)CN)CN(N)CN1(NCN(N)CN(N)CNCNCOCOCOCOCOCOCOCO 

CN  cOrh»OOiOOiOO»OOiOO»OOiOOlOOCN 
cOCOcOtOrhrh«OiONONO  t^  t^  OO  00  ON  O\  O  O  hn  M 

8iOO»OOiOOiOOiOOiOO»OOiOO»OO»O 
cO    t>s*    O     *^"    t>s*    hH     ^h  OO     hH     10  OO     CN|     iO    ON    OJ    \O     ON    to  ^O 

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(N     cOrh«ONO     t^.00     ONO     hn     rj     cOrh»ONO     r^OO     ONO     hn 

hHhHhHhHhHhHhHhHCN)CNCN)CNCN)<NCN)CNCNCNCOCO 

8100   >oo   100   100   100   too   100   100   100   *o 
cOt~»O     rht^>hH     rhoo     hn     lOOO     CN     lOONONO     ONCONO 
hH     hH     hH     CN     d     CN     cOcOcOrhrhrhiOiOiONONONO     f^t^. 

CN  -^-NOOOOCN  rhNOQOOCNi  rhNOoOOn  rhNOGOO 
CN  CN  (N  CN  cOcOcOcOcOrhrhrhrhrhiOiOiOiOiONO 

CH  cOrhiONO  t^.OO  ON©  hn  (N|  cOrhiONO  t>.OO  ONO-hH 
ON  ON  ON  ON  ON  ON  ON  ON  OOOOOOOOOOhHhH 

O     »OO     lOOiOO     iOO     iOO     iOO     »OO     iOO     IOO     iO 

O  co  t^*  O  ^h  t>*  hH  ^h  oo  hH  10  oo  CN  10  ON  04  NO  ON  co  NO 
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hH       COlOt~>.ONhH       CO»Ol^.ONhH       COlOt^ONhH       CO^OO       O 

CN  COrh»ONO  t>.00:O^O  hH  CN  cOrh»ONO  f^OO  ON©  hH 
1s*.  t^»  t^*  t^»  t^*  t^*  t^*  t^»  OO  OO  OO  OO  OO  OO  00  OO  00  CO  ON  ON 

8iOO    iOO    »OO    «OO    iOO     iOO     iOO     iOO     iOO    *O 
COI^O     rht^hH     Tj-OO     hH     lOOO     M     IOON^NO     ONCONO 

i^i^r^cooooooNONONOOOhHhHhHcvicNCNcoco 

O     CN     rt-NOGO     O     CN     rhNO     ONhn     cOiOr^ONhn     coiOt^ON 


hH     CN     CO    rh   »O  NO 

NO     NO     NO     NO     NO     NO     NO     NO     NO 


t^  00     rh    O     hH 


looiooiooiooiooiooiooiooiooio 

cOt>-O     rh   t^    HH     Thoo     hH     lOOO     CN     lOONCNNO     ONCONO 
O     O     hH     M     hH     CN     CN     CN     cOcOcOrhrhrhiOiOiONONO 

§rhNOOOOCN  rhNOOO  OCN  rhNOOOOCN  rhNOOO 
OOOhHhHhHhHhHnCNl(N(NlCNlCOCOcOCOCO 
OOOOOOOOOOOOOOOOOO 


CN|     cOrhiONO    l^.oo    ONO    hn 
cocococococococorfrh 


282 


APPENDIX 


TABLE  23 — WIND  PRESSURE — POUNDS  PER  SQUARE  FOOT 


Ind 
Vel 

0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

o 

o  104 

O  144 

O  IQO 

O  24^ 

O  T.OT, 

10 

0.369 

0-433 

0.5II 

0.586 

0.666 

0.762 

0-853 

0.949 

1.05 

1.16 

20 

1.27 

1.38 

1-50 

1-63 

1.76 

1.90 

2.04 

2.19 

2-34 

2.48 

30 

2.64 

2.  Si 

2.98 

3-14 

3-32 

3-50 

3-67 

3-87 

4.04 

4.24 

40 

4-44 

4.64 

4.84 

5-07 

5-27 

5.5i 

5-72 

5-93 

6.18 

6.40 

50 

6.66 

6.89 

7.12 

7-40 

7.64 

7.88 

8.14 

8-43 

8.69 

8.95 

60 

9.22 

9-49 

9.76 

10.  I 

10.4 

10.6 

10.9 

II.  2 

II.  6 

II.  9 

70 

12.2 

12.5 

12.8 

13-1 

13-5 

13-8 

I4.I 

14.4 

14.8 

15-1 

80 

15-5 

15-8 

16.2 

16.5 

16.9 

17-3 

I7.6 

18.0 

18.4 

18.8 

QO 

TQ  2 

The  foregoing  are  calculated  for  wind  pressure  on  plane  surfaces  at  right 

B 

angles  to  the  direction  of  the  wind  by  the  formula  P  =  .OO4  —  SV2.     P  = 

pressure  in   pounds  avoirdupois;    S  =  surface  in   square   feet;    V  =  true   (or 
corrected)  velocity  in  mi  per  hour;    B=  height  of  barometer  in  inches. 
Corrected  velocities  indicated  for  Robinson  anemometer  for  above  table. 


Tnd 

0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

Vel 

o 

t;  i 

6  o 

6  9 

7-8 

8  7 

10 

9.6 

10.4 

ii.  3 

12.  I 

12.9 

13.8 

14.6 

15-4 

16.2 

17.0 

20 

17-8 

18.6 

19.4 

20.2 

21.0 

21.8 

22.6 

23-4 

24.2 

24.9 

30 

25-7 

26.5 

27-3 

28.0 

28.8 

29.6 

30-3 

3I.I 

31.8 

32.6 

40 

33-3 

34-  i 

34-8 

35-6 

36-3 

37-1 

37-8 

38.5 

39-3 

40.0 

50 

40.8 

41.5 

42.2 

43-0 

43-7 

44-4 

45-1 

45-9 

46.6 

47-3 

60 

48.0 

48.7 

49-4 

50.2 

50.9 

51-6 

42-3 

53-0 

43-8 

54-5 

70 

55-2 

55-9 

56.6 

57-3 

48.0 

58.7 

59-4 

60.  i 

60.8 

61.5 

80 

62.2 

62.9 

63.6 

64-3 

65.0 

65-8 

66.4 

67.1 

67.8 

68.5 

QO 

6Q  2 

REFERENCE  TABLES 


283 


TABLE  24 


Weight  of  a  cubic  foot  of  water 
in  pounds,  av.,  between  the  freez- 
ing and  the  boiling  point.  F 


t° 

Wt 

t* 

Wt 

32 

62.42 

130 

61.56 

40 

62.42 

140 

61-37 

50 

62.41 

150 

61.18 

60 

62.37 

160 

60.98 

70 

62.31 

170 

60.77 

80 

62.23 

1  80 

60.55 

90 

62.13 

190 

60.32 

100 

62.02 

200 

60.12 

no 

61.89 

210 

59.88 

120 

61.74 

212 

59.83 

Volume  of  a  cubic  foot  of  water 
from  temperature  of  maximum  den- 
sity to  that  of  boiling  point.  F 


t° 

Vol 

t° 

Vol 

39-i 

I  .  OOOOO 

131 

.01423 

50 

.00025 

140 

.01678 

59 

.00083 

149 

.01951 

68 

.00171 

158 

.02241 

77 

.00286 

167 

.02548 

86 

.00425 

176 

.02872 

95 

.00586 

185 

.03213 

104 

.00767 

194 

•03570 

H3 

.00967 

203 

•03943 

122 

I.  01186 

212 

•04332 

TABLE  25 — LONGEST  SUMMER  DAY  AND  WINTER  NIGHT  IN  DIFFERENT 

LATITUDES 
Read  down 


1 

Lat 

Mar  20 

Aprs 

Apr  20 

Mays 

May  20 

Junes 

June  20 

Lat 

0° 

12:00 

12:00 

12:00 

12:00 

12:00 

12:00 

12:00 

0° 

10 

12:00 

12:11 

12:23 

12:34 

12:44 

12:47 

12:49 

10 

20 

12:00 

12:21 

12:47 

13:08 

13:27 

13:34 

13:38 

20 

30 

12:00 

12:32 

13:10 

13:42 

14:10 

14:21 

14:27 

30 

40 

12:00 

12:43 

13:33 

14:16 

H:53 

15:08 

15:16 

40 

45 

12:00 

12:48 

1343 

H:33 

15:35 

15:31 

15:40 

45 

50 

12:00 

12:54 

13:57 

14:49 

15:36 

15:55 

16:05 

50 

55 

12:00 

13:07 

14:26 

15:32 

16:28 

16:58 

17:17 

55 

60 

12:00 

13:20 

14:55 

16:14 

17:20 

1  8:00 

18:30 

60 

65 

12:00 

1345 

15:40 

17:25 

19:10 

20:55 

22:40 

65 

66.5 

12:00 

14:00 

1  6:00 

1  8:00 

20:00 

22:00 

24:00 

66.5 

Sept'  20 

Sept  5 

Aug   20 

Aug  5 

July  20 

July  5 

June  20 

! 

Read  up 


284 


APPENDIX 


TABLE  26 — MEAN  BAROMETER  AT  DIFFERENT  LATITUDES 


January  l 

July  * 

Year2 

Lat 

mm 

in 

mm 

in 

mm 

in 

75° 

758-3 

29.86 

758.0 

29-85 

760.0 

29.92 

70 

760.1 

29  93 

757-6 

29.82 

758.6 

29.86 

65 

762.0 

30.00 

757-5 

29.82 

758.2 

29-85 

60 

760.8 

29.96 

757-7 

29-83 

758.7 

29.86 

55 

761  .  1 

29.97 

758.1 

29.84 

759-7 

29.91 

50 

762.3 

30.03 

758-9 

29.92 

760.7 

29-95 

45 

763.0 

30.04 

759-6 

29.90 

761.5 

29.98 

40 

763.9 

30.08 

760.0 

29.92 

762.0 

30.00 

35 

764.8 

30.11 

759-8 

29.91 

762.4 

30.02 

30 

765.0 

30.11 

759-3 

29.89 

761.7 

29.99 

25 

764.0 

30.08 

758-5 

29.86 

760.4 

29.94 

20 

762.3 

30.03 

758.0 

29.84 

759-2 

29.89 

15 

760.5 

29.94 

757-5 

29.82 

758.3 

29-85 

10 

759-1 

29.88 

757-7 

29.80 

.    757-9 

29.84 

5 

758.2 

29.85 

758.5 

29.86 

758.0 

29.84 

0 

758.0 

29.84 

759-1 

29.88 

758.0 

29.84 

Spitaler 


2  Ferrel 


The  difference  in  results  shown  in  the  foregoing  tables  are 
much  too  great  to  be  attributed  merely  to  observational  error. 
Spitaler's  means  were  computed  mainly  from  observations  made 
in  Europe;  Ferrel's  from  American  data.  Means  obtained 
in  latitudes  lower  than  Lat  25°  are  from  unknown  sources.  The 
following  means  were  obtained  at  the  Key  West  Weather  Bureau 
Station,  Lat  24°  33'. 


mm 

in 

mm 

in 

mm 

in 

Jan. 

764.40 

30.09 

May 

761.58 

29.98 

Sept. 

760  .  90 

29.96 

Feb. 

764.08 

30.08 

June 

761.80 

29.99 

Oct. 

760.38 

29.94 

March 

763  •  55 

30.06 

July 

763.10 

30.04 

Nov. 

763-05 

30.07 

April 

762.90 

30.03 

Aug. 

762  .  20 

30.01 

Dec. 

764.34 

30.09 

Mean  for  the  years  1891-1904,  762.69  mm,  30.03  in. 


REFERENCE  TABLES 


285 


TABLE  27 — DETERMINATION  OF  SPEED  PER  HOUR  ACCORDING  TO  DISTANCE, 
IN  SECONDS  PER  MILE 


Time  in 
min      sec 

Miles 
per  hour 

Time 
min      sec 

Miles 
per  hour 

Time 
min      sec 

Miles 
per  hour 

0           20 

180.0 

o        44 

81.8 

08 

52.9 

0           21 

171.4 

o  ;      45 

80.0 

09 

52.1 

0           22 

163.6 

o        46 

78.2 

10 

51-5 

o        23 

156.5 

o        47 

76.6 

ii 

50.7 

o        24 

150.0 

o        48 

75-0 

12 

.50.0 

o        25 

144.0 

o        49 

73  4 

13 

49-3 

o        26 

138.5 

o         50 

72.0 

H 

48.6 

o        27 

133.3 

o         51 

70.5 

15 

48.0 

o        28 

128.6 

o         52 

69.2 

16 

47-3 

o        29 

124.  i 

o         53 

67.9 

17 

46.7 

o        30 

120.0 

o         54 

66.6 

18 

46.1 

o        31 

II6.I 

o         55 

65-4 

19 

45-5 

o        32 

112.  5 

o         56 

64  3 

20 

45  .0 

o        33 

109.0 

o        57 

62  .0 

21 

44  4 

o        34 

105.8 

o        58 

61  .0 

22 

43-9 

o        35 

102.8 

o         59 

60.0 

23 

43-3 

o        36 

IOO.O 

00 

60.0 

24 

42.8 

o        37 

97-3 

01 

59-0 

25 

42-3 

o        38 

94  7 

02 

58.0 

26 

41.8 

o        39 

92-3 

03 

57-i 

27 

41  3 

o        40 

90.0 

04 

56.2 

28 

40.9 

o        41 

87.8 

05 

55-3 

29 

40.4 

o        42 

85-7 

06 

54^5 

30 

40.0 

o        43 

83.7 

07 

53-7 

31 

39-5 

To  find  the  rate  of  speed  in  miles  per  hour  divide  3600,  the  number  of 
seconds  in  an  hour  by  the  number  of  seconds  required  to  traverse  i  mile. 
That  is,  if  i  mile  is  traversed  in  25  seconds,  the  rate  per  hour  in  miles  = 
3600-7-25  =  144  miles  per  hour. 


286 


u 


U 


3   a 
U  2 


U 


en 


APPENDIX 

80     •   o   o '   •   o   o     •   o   o 
vO       •     co   O       •    r>«  vO       •     O     O 


0000     •   o   o 
o    o    o    o     •    o    O 

rj-    O     >-1     O       •     iO    CO 


8O     O     O 
o   o   o 

ro   •<*•  t^  t-» 


;   10  co 


8OOOO       'O 
OOOO       'O 


8.  o  o  o     •  o  o   o  o 
•  o  o  o     •  o   o  o  o 
(s      .osoN«o-ONcor^io 


OOOOOOQO   • 
OOOO  t^»  ^O  O  O 

ioi-ioo»-'Or^oo^t-  • 


oooooooooo 
oooooooooo 

>OO     O     O    O    iOrO'^"t>»t^ 


8      8      8  8 


1  §1 


88888 

cR  10  i-T  to  *^ 

(S     CO    CO    co    cO 


:  8  8. 


:    :  8 


888    :    :  8  8 

r>>   co  (s      •      •    O    O"» 

co  oT  co    ;    ;  >o  >o 


10  »o  »o  t~C     ;    10 


>  8  8  8    :  8 
'  ^i  ^  ^  '  °i 

;     CO    r?    CO      '.     ci 


8888 

M      O     >O    Ol 


O     CO    1-1     ^t-  00 


80   o   o   o   o   o 
o   o   o   o   o   o 
rj-  oo    10  t^.  o    r^  10 


'.     cO    O^    M     cO    »O    1O    Cl 


§00000 
o   o   o   o   o 

O     t>N*  OO    ^O    -O     ^t" 


So   o   o   o   o   o   o 
o   o   o   o   o   o   o 

cOt^-O     **     O^    d     coco 


to  c    vo    t     r    oo    i- 


REFERENCE   TABLES  287 

TABLE  29 — CONSTANTS 

CIRCULAR  MEASURE 

Radius  of  a  circle  in  seconds  of  arc  =  206,264.8062 

Radius  of  a  circle  in  minutes  of  arc  =  3437  -74677 

Radius  of  a  circle  in  degrees  of  arc  57  .2957795 

Circumference  of  a  circle  (360°)  in  seconds  =  1,296,000 

Circumference  of  a  circle  in  minutes  =  21,600 

Ratio  of  circumference  to  diameter  (2ir  R)  =  3  .1415926536 

ASTRONOMICAL 

Calendar  year  =   365  d,  5  h,  48  min,  46  sec. 

Sidereal  year  =   365 . 2563578  days 

Sidereal  day  =   23  h,  56  min,  41  sec 

Mean  solar  day  in  sidereal  time  =   24  h,  03  min,  56.5  sec 

Mean  distance  of  the  earth  from  the  sun  =   92,800,000  miles 

PHYSICAL 

Velocity  of  light  per  second  =  186,337  miles 

Velocity  of  light  per  second  =299,878  kilometers 

Velocity  of  sound  per  second  in  dry  air  at    o°  C  =  1090  V  1+0.00367  t°  C  feet 

Velocity  of  sound  per  second  in  dry  air  at  32  °  F  =  logoVi  +0.00204  t°  F  feet 

/        t°  I        t° 

The  formula  iogo\li  -\ for  Centigrade,  and  1909^  /I  H for  Fahren- 

\       273  \       459 

heit  scales  is  practically  the  same.  For  all  ordinary  purposes  the  value  mo 
ft  per  sec  when  the  temperature  of  the  air  is  50°  F  (10°  C)  or  1148  ft  per  sec 
when  the  temperature  is  86°  F  (30°  C)  will  meet  all  requirements. 


INDEX 


Adiabatic  cooling,  61 
Air  bumps,  58 

—  cataracts,  59 

—  constituents  of,  4 

—  convectional  movements  of,  19,  49 

136 

—  density  of,  44 

—  electrical  conditions  of,  109 

—  expansion  coefficients,  39 

—  holes,  58 

Airman  and  winds,  58 
Alter  shield,  229 

Altitudes,  air  temperatures,  26,  28 

—  terrain,  28,  29 
Ammonia,  7 
Anemometer,  Biram,  239 

—  Robinson,  239 
Anti-trade  winds,  50 
Argon,  6 

Ascending  air  currents,  45 
Atmosphere,  analyses  of,  3 

—  depth  of,  3 

—  unit  of  pressure,  39 
Atmospheric  optical  phenomena,  120 

—  pressure,  39 
Aureole,  121 
Aurora  borealis,  in 
Avogadro's  law,  14 

Barograms,  interpretation  of,  213 
Barograph,  211 

—  adjustment  of,  212 
Barometer,  39 

—  aneroid,  adjustment  of,  207 

construction  of,  206 

engineers',  209 


Barometer,     aneroid,     Goldschmidt, 
207 

pocket,  209 

recording,  210 

Barometer,  mercury,  193 

adjustment  of,  198 

construction  of,  195 

fixed  cistern,  196 

Fortin  cistern,  195 

observations,  202 

rules  for  care  of,  200 

scales,  201 

Beaufort  scale,  239,  240 
Bermuda  high,  41,  145 
Bigelow,  F.,  quoted,  89 
Billow  clouds,  59,  237 
Blizzard,  55 
Bora,  55 

British  thermal  unit,    21 
Brooks,  quoted,  85 

Calm  belts,  52,  53 

Calorie,  21 

Carbon  dioxide  of  atmosphere,  3,  4,  5, 

8 

Cauliflower  cloud,  83 
Ceiling,  atmospheric,  57,  137 
Center  of  mass,  48 
Chinook  wind,  54 
Chlorine,  8 

Cloudbelt,  equatorial,  97 
Cloudbursts,  106 
Cloud  classification,  73,  75 

—  formation  of,  71 

—  heights,  89,  286 
—  records,  85 


289 


290 


INDEX 


Clouds  and  visibility,  140 

—  alto-cumulus,  79 

—  alto-stratus,  77 

—  billow,  75 

—  classified,  73,  75 

—  cirro-cumulus,  77 

—  cirro-stratus,  77,  122,  172 

—  cirrus,  75 

—  cirrus  haze,  77,  122,  172 

—  cirrus  uncinus,  75 

—  cumulo-nimbus,  81,  83 

—  cumulus,  8 1,  83 

—  mammato-cumulus,  79 

—  nimbus,  79 

—  rain,  87 
-  scarf,  87 

—  strato-cumulus,  79 

—  stratus,  83 

—  undulatory,  87 

—  unusual  forms,  87 

—  velo,  71,  140 
Cloudiness,  comparative,  91 

—  distribution  of,  89 

—  minimum,  91 
Cohesion,  12 
Cold  wave,  152 
Colorado  desert,  109 
Condensation,  61 

—  conditions  of,  62 

—  forms  of,  64 
Convectional  layer,  57 
Corona,  120 
Counter  sun,  122 
Counter  trade  winds,  50 
Cross  winds,  237 
Crystallization,  12 

Day,  length  of,  23,  283 
Degree,  determination  of,  177 

—  values,  21 
Dew,  64 
Dew  point,  62 
Diathermancy,  17 
Diffusion  of  moisture,  60 
Doldrums,  52,  97 
Downdraughts,  54 
Ductility,  12 

Dust  and  city  fogs,  127 


Dust  and  condensation,  127 
—  atmospheric,  8,  61 

—  bacterium  content  of,  132 

—  counters,  126 

—  effects  of  on  absorption,  37 

—  electrification  of,  124 

—  nuclei  of  condensation,  61 

—  sources  of,  128 

—  storms,  142 

—  temperature  effects  of,  129 

—  wind-blown,  131 

Earth's  axis,  inclination  of,  23,  25 
Electricity,  atmospheric,  109,  no 

—  extra-terrestrial  influences,  no 

—  of  rain  and  snow  storms,  no 
Ether,  10,  108 

Ether  wave  indicator,  109 
Evaporation,  60 

—  latent-heat  of,  20 

—  measurement  of,  220 

—  rate  of,  60 
Evaporimeters,  220 
Expansion-contraction,  12 

False  cirri,  83 
Fog,  advection,  68 

—  and  visibility,  139 

-  city,  69 

—  formation  of,  68 

—  radiation,  69 

-  sea,  139 

Freezing  mixtures,  256 
Frost,  65 

—  killing,  67 

—  probabilities,  66 
—  warnings,  65 

Gases,  properties  of,  13 
Gravity  defined,  14 
Greely  quoted,  46 
Growing  season  (map),  66,  67 

Hail  stones,  structure  of,  105 
Hail  storms,  bombarding  of,  105 

forecasts  of,  106 

loci  of,  105 


INDEX 


291 


Hail  storms,  notable,  105 

Halo,  121 

Heat,  adiabatic,  20 

—  absorption  of,  37 

—  conductors  of,  19 

—  diffusion  of,  18 

—  in  deep  borings,  17 

—  insulators  of,  19 

—  latent,  20 

—  radiant,  16 

—  sensible,  36 

—  sources  of,  17 

—  specific,  20 

—  units  of  measurement,  21 
Heavenly  cross,  122 
Helium,  7 

Hertzian  waves,  108 
Hoang  River,  131 
Horse  latitudes,  53,  99 
Hot  winds  of  the  Plains,  55 
Howard  quoted,  73 
Humidity,  absolute,  215 

—  and  health,  63 

—  measurements  of,  215 

—  relative,  62,  215 
Humphreys,  W.  J.  quoted,  3,  129 
Hurricane,  West  In'dian,  153 
Hydrogen,  6 
Hygro-autometer,  216 
Hygrodeik,  217 
Hygrograph,  219 
Hygrometer,  hair,  218 

—  Mason's,  216 
Hygroscopes,  215 

—  chemical,  256 


International    Meteorological 

gress,  75 
Isle  Dernier,  153 
Isobars,  47 
Isothermal  layer,  27 
Isothermal  lines,  24 
Isotherms,  January  (map),  30 
—  July  (map),  32 

Jameson,  P.  R.  quoted,  46,  63 
Khamsin,  55 


Con- 


Kimball,  H.  H.,  quoted,  130 
Krakatoa,  eruption  of,  22,  128 
Krypton,  6 

Land-and-sea  breeze,  54 
Latitude  and  temperature,  29 
Lid,  atmospheric,  57,  137 
Light,  absorption  of,  120 

—  diffraction  of,  120 

—  reflection  of,  120 

—  refraction  of,  120,  143 
Lightning,  forms  of,  115 

—  Reeder  quoted,  115 

—  safeguards  against,  1 19 

McAdie  quoted,  232 
Mackerel  sky,  77 
Magnetism,  13 
Malleability,  12 
Matter,  10 

—  forms  of,  II 

—  properties  of,  1 1 
Millibar  defined,  39 
Mistral,  55 
Mirage,  123 

—  and  visibility,  143 
Mock  moon,  121 

—  sun,  121 
Mohave  desert,  109 
Mohn  quoted,  46 
Monsoons,  52 
Mountain  valley  winds,  54 
Mount  Washington,  48 

Neon,  6 

Nipher  shield,  229 

Nitric  acid,  7 

Nitrogen  of  atmosphere,  3,  4 

Northers,  San  Joaquin,  55 

—  Texas,  55 

Optical  phenomena,  atmospheric,  126. 
Oxygen,  4 
Ozone,  7,  8 

Pampero,  55 
Paraselenae,  12 1 
Parhelia,  121 


292 


INDEX 


Photogrammeter,  71 
Pillar  of  light,  122 
Platte  River,  131 
Precipitation,  intensity  of,  226 

—  measurement  of,  97 

—  summer,  107 

Pressure,  actual  and  recorded,  47 

—  and  altitude,  48 

—  atmospheric,  39 

—  barogram  of,  45 

—  daily  ranges  of,  44 

—  diurnal  changes,  44 

—  mean  for  January  (map),  40 

July  (map),  42 

over  earth,  44 

—  measurement  of,  39 

—  semi-diurnal.    Mt.  Vernon,  46 

—  units  of,  39,  193 
Prevailing  westerlies,  50 
Prudden,  T.  M.  quoted,  132 
Psychrometer,  sling,  217 

Rainbow,  122 

Rain  and  visibility,  141 

—  drops,  size  of,  97 

—  insurance  against,  107 
Rainfall,  annual  (map),  98 

—  cyclonic  storms,  100 

-  Pacific  Coast  of  U.  S.,  99 

—  Torrid  Zone,  99 

—  Western  Europe,  99 
Rain  gauges,  Ferguson,  230 

Friez  recording,  224 

installation  of,  229 

Marvin  recording,  223 

pit,  229 

Short  and  Mason,  225 

tipping-bucket,  224 

Weather  Bureau,  222 

Reed,  W.  B.,  quoted,  83 
Return  trade  wind, 
Roaring  forties,  50 

St.  Elmo  fire,  115 

Santa  Ana  wind,  55 

Saturation,  6,  214 

Scud,  81 

Shaw,  Sir  Napier,  quoted,  137,  188 


Simoon,  desert,  55 

—  electrical  effects,  109 
Sleet,  103 

Sling  psychrometer,  217 
Smith,  J.  Warren,  quoted,  119 
Smoke  and  cloud  formation,  57 

—  and  visibility,  142 
Snow,  101 

—  effect  on  visibility,  141 
Snowfall,  distribution  of,  102 

—  measurement  of,  230 

—  of  Pacific  Coast,  230 
Snowflakes,  structure  of,  101 

—  photography  of,  102 
Snow  gauges,  231 
Solar  constant,  21 
Sounding  balloons,  57 
Specific  gravity,  15 

Storm  cards  of  hurricane,  155 
Storms,  anticyclonic,  152 

—  classified,  150 

—  cyclonic,  101,  149 

—  dust,  142,  163 

—  forecasts,  151 

—  form  and  dimensions  of,  156 

—  hail,  105 
-  ice,  103 

—  probabilities,  157 

—  secondary,  158 

Storm  tracks  of  U.  S.  (map),  154 
Stratosphere,  27 

—  humidity  of,  28 

—  movement  of  air  in,  57 
Summer  precipitation,  107 
Sun  pillar,  122 

Sunshine  in  U.  S.  (map),  248 

—  measurement,  250 

—  results  of,  251 
Sunshine  recorders,  242,  245 

Campbell-Stokes,  246 

Marvin,  247 

photographic,  247 

Temperature,  absolute,  177 

—  and  altitude,  26,  57 

—  and  latitude,  29 

—  radiation,  36 

—  and  wind,  35 


INDEX 


293 


Temperature,  anomalies  of,  187 

—  effects  on  civilization,  37 

—  inversion  of,  26 

—  maxima  and  minima,  34 

—  mean  annual,  31 

—  mean  seasonal,  31 

—  normals,  33,  34,  35 

—  ranges,  33,  34 

—  records,  New  York  City,  33 

Cooperstown,  N.  Y.,  33 

Thermograph,  190 

—  high  air,  192 
Thermometer,  adjustment  of,  184 

—  black-bulb,  192 

—  construction  of,  179 

—  graduation  of,  180 

—  maxmium,  183 

—  miminum,  185 

—  shelter  for,  186 

—  Six's  pattern,  189 
-  standard,  181 

Thunder,  causes  of,  114 
Thunder-storms,  112 

—  and  cumulo-nimbus  clouds,H4 

—  forecasts  of,  118 

—  Humphreys  quoted,  113 

—  occurrence  of,  117 

—  pressure  waves,  117,  118 
Tornado,  158 

—  prevalence,  162 

—  regions  of  frequency,  162 
Trade  Winds,  50 
Troposphere,  27 
Turbulence,  atmospheric,  69,  135 

Updraughts,  58 

Velo  cloud,  71,  140 
Vesuvius,  eruption,  83 
Visibility,  atmospheric,  133 

—  factors  of  observation,  134 

—  forecasts  of,  144 

Warmth,  diffusion  of,  24 
Washoe  Zephyr,  55 
Waterspouts,  163 
Water  vapor  of  the  air,  5,  9,  214 
Weather  folklore,  167 


Weather  folklore,  annual  indications, 

173 

barometer  indications,  167,  169 

cloud  indications,  172 

—  humidity  indications,  170 
plant  indications,  173 

—  forecasts,  164 

—  information,  147 

—  map,  daily,  146 

West  Indian  hurricanes,  101 
Wet-bulb  thermometer,  215 
Whipple  quoted,  47 
Whirling  table,  217 
Whirlwinds,  desert,  163 
White  squalls,  163 
Wind  direction,  234 

—  recording,  241 

—  velocity,  measurement  of,  56,  237 
Winds,  Atlantic  Oteean,  53 

—  Pacific  coast,  56 

—  United  States,  51 

—  Prevaling  Westerlies,  35 
Winslow,  C.-E.  A.  quoted,  63 
Winter  storms,  101 

Xenon,  6 

Zero,  absolute,  178 

—  Centigrade,  178 

—  Fahrenheit,  178 

—  Reaumer,  178 
Zones,  climatic,  24 

-  light,  24 

Reference  Tables 

C.  G.  S.  units,  253 
Electricity,  units  of,  255 
Magnetism,  units  of,  255 

TABLES 

i.  MISCELLANEOUS  TABLES: 
Atmospheric  equivalents,  260 
Capacity  equivalents,  259 
Cubic  equivalents,  259 
Linear  equivalents,  258 
Miscellaneous  equivalents,  261 
Precipitation  equivalents,  260 


294 


INDEX 


MISCELLANEOUS  TABLES — Continued 
Velocity  equivalents,  261 
Weight  equivalents,  259 
Work  equivalents,  260 

2.  Inches  to  centimeters,  262 

3.  Centimeters  to  inches,  262 

4.  Cubic  inches  to  cubic  centimeters, 

263 

5.  Cubic  centimeters  to  cubic  inches, 

263 

6.  Miles  to  kilometers,  264 

7.  Kilometers  to  miles,  264 

8.  Feet  to  rneters,  265 

9.  Meters  to  feet,  265 

10.  Barometer  inches  to  millimeters, 

266 

11.  Barometer  millimeters  to  inches, 

267 

12.  Barometer    inches    to    kilobars, 

268-271 

13.  Miles    per    hour   to    meters    per 

second,  272 

14.  Meters  per  second  to  miles  per 

hour,  273 


1 5.  Comparison  of  temperature  scales, 

274-276 

1 6.  Degrees    Fahrenheit     to     Centi- 

grade, 277 

17.  Boiling  point  of  water  F,  278 

18.  Boiling  point  of  water  C,  278 

19.  Quantity  of  rainfall  in  cubic  feet, 

279 

20.  Depth  of  water  to  weight  of  snow, 

279 

21.  Weight  of  cubic  foot  of  saturated 

vapor,  280 

22.  Expansion  of  air,  32  °-2 12°,  281 

23.  Wind  pressure,  pounds  per  sq  ft, 

282 

24.  Weight    and  volume  of  cu    ft  of 

water,  32°-2i2°,  283 

25.  Longest  summer  day,  Lat  o°-66°, 

283 

26.  Mean  barometer  at  different  lati- 

tudes, 284  t 

27.  Speed  per  hour,  285 

28.  Altitude  of  clouds,  286 

29.  Constants,  287 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below,  or 

on  the  date  to  which  renewed. 
Renewed  books  are  subject  to  immediate  recall. 


WNr 


LD  21-100m-6,'56 
(B9311slO)476 


General  Library 

University  of  California 

Berkeley 


YC  'I085P 


465041 


UNIVERSITY  OF  CALIFORNIA LIBRARY 


